Self-Conscious..?

My boyfriend and I have been together over 2 years now and in that time, he’s gained some weight. I don’t care all that much, I love him no matter if he’s a small or an XXL, but lately its been bothering him, especially during sex. Last night he finally told me that it really does make him feel bad because he doesn’t want to do missionary, because he feels too heavy and that I dont like it. I tried telling him that he doesn’t hurt me by being on top, and that I enjoy it because it’s a position that makes me feel safe and loved (kind of weird, but its missionary, and it’s warm and loving lol). He was still so sad and I just feel terrible that what I’m saying isn’t working. I tell him all the time how good he looks, especially when it comes to clothes shopping since he’s gone up a size, and I know that there’s only so much I can do, but any tips to help him feel a little better? I love him and I enjoy everything we do in the bedroom, but to be quite honest, me being on top all the time is getting a little...repetitive.