Gaining

I mean there's not really much you can do except give it time and see how he reacts in the future. I mean I suppose you could make some passing comment on your weight to prompt a certain response and gauge their reaction.

But ultimately being open and honest with one another is probably for the best. But apprehension when it comes to this fetish is understandable.

Is here looking at other heavier ladies, when
they pass by? This might indicate he likes them
and you.

There are ways to get his opinion without saying too much. You can say that your clothes are getting tight, and see if he comments.

If he comments on you being thick again, you could say something like "I know, right? But I kind of like it. I've got more curves, and I just feel like I have a little more presence than when I was so thin."

Which isn't telling him your fetish, but it tells him you like what has happened so far and are happy with your body, and that is a start of the conversation.

Basically, sometimes you just need to get a foot in the door, you don't have to smash the door into their face.

I think you should just be up front about it. You don't have to tell him it turns you on (though I think if you do and he is understanding it might bring you closer together) but you could simply say something like "hey I realize I've gained some weight weight and I'm wondering if it turns you off?" From there you can ask further questions. Like if he says he is into it you could ask him if he there would be point it would start bothering him and so on.

Seems to me if you are into this and seriously want to gain as a life style you need to find out if he's truly OK with your interest. The best way to find out is to be up front and honest about it with him. He ought to have the option of either accepting or rejecting your fetish if it is important to your sex life and the relationship is serious - and it very much sounds like it is.
Of course, you're afraid of losing him if he's either not into it or unwilling to put up with it but if you end up together and later decide you want to be open about it and he doesn't approve, he may take off then, perhaps leaving you after having started a family.
Shouldn't he know who you really are, and you him, before you either marry or permanently commit?
The other option is to keep your interest hidden for life. I'm sure plenty have done, and are doing today, just that.