General

Honesty is the only way to go about this. Keeping it locked up, and torturing yourself will only make it worse for yourself.

In a relationship you need trust, and the ability to confide these kind of things to one another. If you can't tell him how you feel that's a bad sign.

It's great that you want to be there for him and make him feel attractive. But if he isn't into it now, unless you talk to him, I doubt it will work out well. Even then it seems like it will be a bit of a stretch given what he thinks about this...

Eh I feel like people imagine the extremes in these sorts of things. It might gain acceptance eventually...

But not telling him... If he ever found out any other way, how would that look? It's best to hear it from you, honestly and truly, than some weird circumstance like him seeing some email you write.

or something like that... Life always has some way of bringing out the truth.

Hiding things even if they aren't big of a deal creates a lack of trust between two halves. If you truly love and trust him, he should know how you feel to some degree. If you are truly happy with him at his current size... maybe come upfront as a chubby chaser and tell him that you find him attractive now, and that you don't want him to feel like he needs to hide his body from you.. Or tell him the second part...

It's scarier to admit it to someone you love and are in the relationship with, than it is to be upfront with those that would be around no matter what truth you told them (within reason)

Positive encouragement! Make sure you tell him regularly that he looks very good, that you love it when a man is not too skinny, that you love his belly, that you think it is cute, etc. Maybe at some point he will make the click and let you love all of him.

I fully understand you. I love my gf's body, but would love it even more and sometimes fantasize about her being quite a bit heavier. But I love it even more that she is a very amazing person, and that is more important to me than having the ideal SSBBW body.

I think there are 2 issues at play in your situation: your insecurities about what you like and his insecurities about his body.

About your own insecurities: There is nothing wrong with liking his body but fantaszing it was bigger. What is important is that you maybe not get maximal sexual pleasure, but enough to get you going. If he would let you touch him the way you would like and if you could let your FFA side more out, would that be enough for you? Tell him that you like his belly, that you think it is cute and like to touch it, but do it in a natural and gentle way.

That brings us to his insecurities: Don't bluntly tell him that you fantasize about 400 lbs guys, because if he is not happy with his current body, he might be repulsed by the idea. There is no need to label yourself as an FFA. But do try to convince him slowly, step by step that you like his body, that you don't mind a big belly and some fat rolls on a man and that you think he is very pretty. Hopefully, as time passes by, he will let you appreciate his body more and maybe start to like it. When he is more comfortable, you can tell him that you always liked bigger men.
Will he ever get to the point of liking his body? Impossible to tell. Some people get there easily and other people never do.