How do I tell him??

To be honest, if you have tried hinting, cooking better, and suggesting working out together, I think it's now time to just tell the truth. I think you not wanting to be intimate with him is probably hurting his feelings more than just telling him that you just don't feel as sexually attracted to him as you once were before he gained weight. I am a firm believer in honesty, even if its a little brutal. It serves no one good to beat around the bush. You'd be surprisd at how well people can take the truth sometimes. Maybe it's something he already knows and feels but doesn't want to admit.