I agree about this being a dealbreaker if he's unwilling to come around. I've dated picky eaters before and not minded, as long as it was reasonable. And while I'll eat and enjoy just about anything, I'm not a foodie by any means. I tend to fall back on a few staple meals when I'm eating alone, but they're things like stir fry, salads and crock pot meals.

But man, this is so extreme that it will bleed over into everyday life. You'd have to pick every restaurant based on whether they have grilled cheese, you couldn't go over to friends' houses for dinner, forget about ever going out to brunch or breakfast--when you travel, you'd have to make sure that your hotel offered Froot Loops at the continental breakfast, or bring your own which is just embarrassing. And that's not even getting into what would happen if they have kids and poor OP is over there trying to get the kids to eat their veggies while Dad chows down on 5 grilled cheese sandwiches, or the inevitable health issues that are going to plague him.

It could just be that "I'm an adult, I can do whatever I want!" attitude a lot of college kids have gone amok, and he might grow out of it. But yeah, if I were in the OP's shoes, I'd talk about it seriously with him and break up if he didn't take it seriously and change his ways at least a little bit.