<human>: Give me the gist of the text in a nutshell

This is a throwaway.
I have been with my boyfriend for almost 4 years, we dated all throughout high school. When we got together we both did multiple sports and both were bean poles. 4 years down the road, he has gained some weight. 

I am 18f, 5'7 and weigh 120, he is 18, 5.8 and weighs 180(?). 

It really isn't that big of an issue, but I feel like I look and actually am healthier/ more attractive than him. He has a belly and he has a chubby face, and complains about his belly all of the time. He beats himself up about how I'm "too sexy for him" or how he's "like a slug". Obviously I'm trying to be kind and supportive by telling him it isn't that bad, because in reality it isn't, I just wish he wanted to lose weight. He's always been really insecure, so it makes it very difficult to give pointers/send messages about this kind of thing. 

I look back at pictures from a couple years ago and he was very fit and it makes me miss it. He doesn't ever want to do anything outside, when I would love to go for a walk or even sit outside at a restaurant. When we plan to hang out together, he often wears sweatpants (he used to be very fashionable) while I'm wearing a dress or something nice at least. It isn't affecting our sex life TOO much,  but it definitely used to be better. 

I love him so much and I plan to spend the rest of my life with him, I just want to find a way to coax him back into his old habits. Is it my fault for telling him its not that bad? Does he feel like he doesn't need to try to look good? Thanks for reading.

Also thought I should add, a diet won't work. He pretty much hates every food under the sun. Only veggies he likes are corn and broccoli, only fruits he likes are apples, and proteins are steak, nuts, pork and chicken. Doesn't like his food touching either, pasta made with cheese and noodles is his favorite dish... Or BK chicken fries.
<bot>: My boyfriend has gained 50+ lbs in the 4 years we've been together. It isn't a huge deal, I just needed advice on how to talk to him about losing weight.