Rachel,
As a picker eater growing up, I was amazed to find out as an adult, and after visiting an allergist, that I have a TON of food allergies! So if he's this picky, it may be because food literally makes him feel sick when he eats it, and he's sticking to stuff that doesn't make him feel bad. My first recommendation would be to take him to an allergist and find out what food allergies, if any, he has. Then later when you try new foods on him, you can honestly say "Hey, it won't kill you to try it." ;)
(And there are some things that allergists won't point out that are true - like I can't eat pork because all the pork products in stores use Sodium Nitrates to preserve the meat, which gives me migraines... so just because the allergist doesn't mention something, doesn't mean there isn't an underlying issue to why he's turning his nose up.) Most of the things I'm allergic to taste terrible to me, like corn. So nature does try to protect us! :)
Once you know for sure what is safe to feed this guy, I would recommend making meals for him you know he already likes, and adding one new thing to the meal. So he likes grilled cheese sandwiches? Great! Try adding green beans to the meal. They're super bland if you don't add anything to them! ;) Try foods first without adding anything to them at all - see if he likes the taste of the food itself. Most people add bacon to green beans - to me this is disgusting! But the beans by themselves I can eat a whole mess of! :)
I'd say try getting him to eat a cheese omelette for breakfast (unless he's allergic to eggs - then it's an egg-white omelette with cheese... I'm allergic to egg yolks, so I know!). It's super easy for you to add the veggies you like to the omelette and leave them out when making it for him, and eggs are very healthy to eat, esp if it's an egg-white only omelette (heck, with all that cheese you might want to leave the yoiks out anyway). ;)
Once he likes eggs, then try expanding his breakfast by adding a different fruit. First week - grapes! (Since you know he likes them.) Next week? Maybe try grapefruit halves, then the next week strawberries, the next week blueberries... etc, etc. Just give him a whole week with each new fruit, telling him it takes 3-5 times eating a food before you can really know if you like it or if you're just rejecting it out of habit. This way, you know he gets fed with the eggs, and the fruit is a bonus. And you can try varying the fruit over the course of the week (ex: grapefruit plain day 1, with sugar day 2, with grapes on the side day 3 - mixin' it up! - fresh squeezed grapefruit day 4, and then maybe a different variety of grapefruit on day 5, 6, and 7 since there's more than one type of grapefruit out there!).Of course, if you find he likes a particular combo, like grapefruit with sugar, reward him by serving that the rest of the week so he can get used to eating it and then move on to a new fruit the next week, knowing you can come back to grapefruit safely in the future. ;)
For years, my dad would only eat pancakes for breakfast (and sometimes lunch and dinner), but now he'll eat almost any fruit out there for breakfast. My mom introduced him to fruit this way - adding it as a side dish to a food he already liked.
Do the same at dinner time. He likes cheese pizza? Great! Add a different topping to one slice each week for him... a little spinach one week, a little mushrooms the next week (I'd stick with white mushrooms - they're the most plain flavor-wise, and if you could put fresh-cut white mushroom on the pizza right before cooking it, it'll be dry not slimy, making him more likely to enjoy it). Maybe pineapple one week, etc, etc. Let him eat one piece of plain cheese first, to whet his appetite, then have him eat the pizza slice with one topping, and if he doesn't like it, he can fill up on plain cheese the rest of the meal. If he does, then you can make him two slices with the new topping the next few days. Get him used to eating it! :)
After a week of pizza, maybe try a week of cheese burgers. Try one new food on it for him each week, and you can buy a variety of foods for yourself (if you're buying from restaurants) or make other stuff for yourself than just cheese burgers (show him it's possible to eat other things! maybe sandwiches, or souop, or chicken fingers, or whatever you want). The temptation will be if he likes tomatoes on his cheeseburger, to keep them on and add pickles the next week. I'd recommend just one food at a time, tomatoes one week, pickles the next week, lettuce the next week, etc. If you want to speed things up, you can do cheeseburgers for lunch and pizza for dinner (or vice versa). Just recognize that preparation of foods make a big difference! Some people like tomatoes sliced, some like them diced, some like them fried (eww to me! but to each their own), some like them julien, so try varying how you present the tomatoes. Same with pickles - there are 100s of types of pickles on the market, so try a different type of pickle each day til you find one he likes! (You might even try cucumbers - yum!!! because I hate vinegar - LOL). Some people like a leaf of lettuce, some like it shredded, some like romaine, some like iceberg, etc, so experiment each week with the new food you're putting on, and don't mix them. I love onions carmelized, grilled, raw, purple, white, (but not yellow), diced, fried, almost any way at all! You might find a way of preparing them that he likes. ;) Once you've found a few foods he likes (if you do), then you can try having him eat them at the same time on the burger. He might like that, he might not. He may even find he likes the taste of them better if there's no cheese! (But that's once you get to more advanced experimenting levels). ;)
Meatloaf ... wow! Everyone on the planet seems to put ketchup on meatloaf. To me - barf! The nice, plain way I like meatloaf is the following recipe: one package Lipton Onion soup mix (you can use fresh diced onions and a half cup beef stock as a substitute, or a beef bullion cube and onion and a little water if he doesn't like the soup mix), one whole egg (cooking it well enough to get the meat thoroughly cooked destroys the egg protein in the yolk that I'm allergic to, so it's safe in this instance), mixed well with the meat, shaped into a loaf, cook and serve. That's it! No breadcrumbs, no maters, and I like to eat meatloaf with mashed potates that have nothing in them but a couple pats of unsalted butter (I HATE salt, and that's because I'm allergic to iodine). No milk, no gravy, no salt or pepper, nothing else! (He might like cheese on top though - LOL). So if he likes bland, you can't make it much blander than this! Oh, and I usually eat my no-thrills green beans with this meal too. ;)
Change is hard, but often people take to sweets pretty easily. I'd only feed him one dessert a day, and only after you've found breakfasts and lunches and dinners you can feed to him, so it's the dessert that's changing. Figure out if he has texture aversions. My sister-in-law won't eat boiled okra or mushrooms "because they're slimy". I love both, but hated hard candies until just a few years ago (I love wintergreen lifesavers now). LOL
Try different pies, different cakes, different cookies. I hated oatmeal raisin cookies as a kid, and love them now, so taste changes over time (I prefer oatmeal chocolate chip cookies though). ;) Oddly, despite the fact that I prefer bland food, I love carrot cake! It's the only thing in the whole world I like cream cheese on. He might like it too, once you've expanded his food horizons some. ;) ... Personally, I hate cheese cake, but he might love it since he's a cheese guy.
There are truly hundreds of ways of making chocolate cake out there. Since I'm allergic to wheat, I love flourless chocolate cake, but I'd say don't give up on chocolate cake if you like it and he doesn't. Find a chocolate bar he likes the taste of (if possible) and make a homemade cake using that chocolate. Some men just hate chocolate though, and you can't change that. So see if he likes strawberry cake, yellow cake, white cake, cherry pie, pumpkin pie, apple pie, etc. Make or buy ones you actually like the taste of to start, so if he takes to them at least you're on the same page. ;) The fewer ingredients that are in these recipes, the better your odds of finding one he'll like (the flourless chocolate cake I like only has three ingredients - eggs, butter, and sweet chocolate). I'd say, keep it simple! :)
Picky eaters can be a handful, but we know what we like and we're very, very loyal, which makes us good mate material. :)