Just stop having sex with him and refusing his advances and/OR try to get him to work out and eat better with you. If he asks why you've stopped having sex, just tell him the reason why and reiterate that you still care about him but aren't as physically attracted due to his weight gain.
Very subtly to start with its a big hit for a man to take and your right to intervene. For his own health and your relationship. If straight talking does get you somewhere then try the seductive root. At certain weight loss goals over him something sexual when the target is reached. Building up to something very special at the ultimate target. Good luck.