General

Hello. I'm a domme female feeder who has a male feedee. Not quite the same situation, but some thing transcend gender.

Honestly, it sounds like your bf needs therapy. This isn't a healthy response and I don't think he is capable of working through it himself. And if you two let this fester, it will cause relationship issues later on down the road.

While I am more than happy to fatten my feedee, I put his comfort and happiness above mine. So even though I am in charge, it is ultimately his choice. This means that if he wants to maintain or lose weight, I will happily support him

It's great that he isn't pushing you to be fat. But that's only one side of the coin. And he, as a grown man, shouldn't sulking in the corner at the very ideal of your losing weight. It doesn't sound like you are trying to lose weight, but how would he react if you lost even one pound?

Again, at this stage, this won't break your relationship. But I must stress he gets therapy. He needs to work on himself and you can't fix him - even if you gain a hundred pounds.

I understand where he is coming from, I hated it when my wife dieted down for our wedding, but agree with the above that he needs help. He's having obsessive thoughts, and a good therapist should be able to help with that, maybe CBT or something. His desires will be there for life, nothing that can be changed there, but this anxiety being caused by them is something that good therapy should be able to address

One thing to think about though is even if his anxiety and obsessive thoughts get under control, his sexuality is still going to be wired to wanting to see you gain weight. If you don't want to, then that could be an issue longer term if he is feeling constantly sexually unsatisfied. Not sure what to suggest here. Some people report finding role play to he enough for them, but I know for myself that doesn't do much.
One pretty out there suggestion, which may not be relevant to him at all, but I guess is worth throwing out there, is that it's actually very common for male feeders to have feedee tendencies too aka the desire to be fattened by a partner themselves. If he is also into that and you don't mind him gaining weight, you could ask him if that would scratch his weight gain itch if you're not going to get fat. Would be worth asking him as long as you're ok with it.
I know for myself, I definitely prefer my wife being fat, but now that I'm gaining myself I care far less when she loses some weight here and there