General

First of all, calm down, it's okay, I understand the situation, I have been in a relationship with someone in a very good shape and I always encouraged her to eat more and more, of course she never wanted. It frustrating, you love him a lot but deep inside you know that he isn't perfect for you, because well, I think you like him a lot, but sexually he will never please you at all.

If I were you, I will keep feeding him, buying him food, take him to eat his favorite foods, and talk with him about his extra weight, ask him if he is comfortable with it, if he have plans to lose some weight or if he is trying to lose it right now. If he says that he isn't comfortable with this extra weight and he want to lose all that extra fat, sorry but I don't think he will change his mind.

But if he is comfortable with it just keep going, and in some time, when you have enough confidence to tell him, tell him all about feederism. If he leaves you after knowing about it, he is an asshole, if he likes the idea, great for you, but if he just don't like it at all and he want to keep being with you just talk with him, maybe he doesn't want to get any fatter, but maybe you can feed him sometimes without the necessity of making him bigger.

Finally, don't feel guilty because you want to make him fatter, you doesn't choose to like this, anyone choose it, if he likes the idea of getting fat and he doesn't have any trouble with some extra pounds its okay, you don't force him to get fatter, he decided that he likes being like that.

Sorry if I made it so long and I hope you can find a solution!

Jamie is very much correct.

Hope it all works out for you.

You shouldn't ever feel guilty for what you want, just for what you do. That said, you have been encouraging him without letting him know that you will be doing that, so maybe feeling a bit of guilt would be reasonable (but not a lot, you've not enabled him any more than a lot of other people would).

Without telling him the heart of your kink, you can start sort of nibbling around the edges of it. Let him know that you think he looks better than he did in highschool. Tell him that you like having some softness to grab. Tell him that you love seeing him enjoy his food. Tell him that you love providing food for him in general. Tell him that you love feeling his weight on top of you.

Don't do it all at once. Think of other things you could tell him. Overall make it clear enough that you are not wanting him to lose weight.

Then as Jamie said, try to relax and not over think it. Give things some time, see how it goes.

Everything can be seen through the lens of manipulative if you study hard enough. Like your partner to save money, convince him to eat lunch in his office by packing him a favorite lunch. Are you manipulating him to do what you want (save money) rather than enjoy the lunch out with friends?

I am going to stick with my previous take,...

encourage but not force

ease him into knowing your likes by teasing as was suggested.

Reward him for doing what you like, it is after all how the world has always worked.

Hope it works out for you.