Honestly, how would you want to be told? Would you not pop an attitutde? I know I would and I would lose my religion cursing a fool up and down about it, as if I didn't know that I had put on weight.
Just keep offering him to work out with you. He has to make the decision about losing weight, not you.
What are you going to do if he doesn't lose the weight? Are you not going to marry him? Marry him and still bug the heck out of him because of his weight? For Pete's sake, you said the man had medical issues.