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.