Perhaps you should sit down with your boyfriend and just stress that, while you want to make him as happy as possible, you hope that he wants to do the same for you in return.

Let him know that you're concerned about your physical image and that you're having trouble meeting his needs. Weight-gaining is a very giving thing because it's difficult to drop it all afterwards, whereas it's easy as pie (unsure if pun intended..) to put it all on.

Why don't you look into alternate means? When you mentioned pillow-stuffing to give the illusion that you've gained weight, it sounded very similar to me because my main sexual preference is body inflation. I'm unsure myself how I've become interested in it (a lot of the reason I'm here) but you can look into inflatable suits and balloon-stuffing to essentially provide the same illusion that you're gaining weight.

A lot of people in the inflation scene invest in rubber/latex suits to wear underneath clothing and then inflate. Perhaps that's something you could look into? I've tried it myself and I've found it to be quite a therapeutic feeling sometimes, but it would definitely be something to look into for you guys and you may find that in-between where you're both happy. You can keep a physical image you're happy with and your boyfriend can still become aroused by the sight of you becoming bigger (or inflating).

That's just my two cents, though.

I hope you pair can come to a good conclusion.