Please tell me I’m not a bad girlfriend

My boyfriend and I have been together for almost 4 years and we have 2 beautiful little girls. Over the years I notice he overeats bad, and he constantly complains about his weight he is 237 pounds, and both is his parents are diabetic I believe. Well I really want to help him and encouraging him not to overeat but I don’t want to seem like asshole when being honest with him when he does or telling him, “No you don’t need a second or third plate.” Something alone those lines. Should I help him or not say anything?