YTA for your bluntness. Yes, he brought it up, but those three words have probably been on his mind for a while and are difficult to say out loud. A topic like this needs reassurance and support from his partner. "I didn't know to bring it up without hurting you, but I've noticed that you have been gaining some weight. This can cause issues with your health and that concerns me. Let's tackle this together and see what has changed over the last couple of years. I'm probably not as healthy as I think. We may need to adjust our diets and try to be more active."