Gaining

All you have to do is cook for the guy, I'm sure he likes to eat! Try even going as far as to fill his plate for him! Don't go overboard like needing side-boards or a giant platter...but subtle increases over time and dessert will pack it on him quick!
Also, it wouldn't hurt to give him a little extra dessert after his first dessert, "if ya know what I mean".

From your recent picture and your comment below- It would seem that you have gained a good bit lately. And if that continues ,in another month or so your own weight gain will make a most convenient conversation starter.And it make conversation on the subjectuneeded. Maybe he'll see you relaxed and happy-and he might relax too.He certainly shouldn't be as worried abour it If he gains then he can just say that you're a happy couple getting chubby together.

Yes, mention it.

I think you can share your thoughts with him quite easily. Any time I was with a woman who mentioned her weight, I always express how well she wears the few extra pounds and that not matter how much she gained--or lost--she would still be beautiful, engaging, interesting and just plain fun to be around. I've also made the mistake of focusing too much on gaining. Nonetheless, every woman I've been in involved with has gained weight. Doesn't hurt that I'm witty, charming, kind, lightly sarcastic and a good cook and restaurant critic