I need SO advice

So my boyfriend and I have been together 7 months (almost) but if you see us, we seem like we've been together for years. He's my rock, my everything, and my only love. Recently he's been gaining weight and gotten very self conscious about it. I keep saying I'll go to the gym with him, make healthier foods, and even that no matter what he looks like, I'll still love him. He just gets sadder and sadder when he talks about it or asks me about it. Anyone who has been through something similar or can give me word of advice? TIA