Gaining

My thought is that you're 1.Quite Beautiful. 2.If someone is dating you they feel the same as myself and pretty much everyone on this site.
So I would advise you to be honest about everything. Or fish for clues. Like saying you have been fat your whole life. Which is true. But the fishing part would be to say how you have fought hard to only be This Big. Also the truth. And another truth. You would Love to Not Worry and just enjoy yourself. Also the truth. His /her reaction will tell you all you need to know. Good luck.

When you are dating an FA, chances are big that he is at least open to it. I don't want to say that most FAs are feeders, certainly not, but as a non-feeder FA myself, I have the feeling that a feedee better understands our FA feelings. A feedee loves her curves in the same way as a FA does. And most FAs even if they are not feeders, are not against some weight gain.
Bring it up gradually tough. Explore what he likes, give subtle hints, see what his reaction is when you say you have gained some weight, etc.