Gym buffs tend to see weight gain as an indicator of psychological flaws. I've read a little into it and they say all kinds of things about overweight people - overweight people are letting themselves go, they have no willpower, they don't care about their body, etc. It's even worse when the gym buff is a guy because guys are more visual. A gym buff won't feel any empathy with analogies where he develops a medical condition or gets into a serious accident, because to him, weight gain is a choice people make through negligence, stupidity, or apathy... nevermind that your priorities might be as a mother might be somewhat different than his.

What I would suggest is to do things that show him that you're not "letting yourself go". Stay on top of your appearance like you would if you were single. Be lighthearted, funny, and sexual, even if he doesn't appreciate it at first. Keep your self-esteem high by setting goals and accomplishing them. Enlist his help in losing weight - it goes faster if you have a partner, and I'm sure he knows the science behind it. Once he sees that a valuable woman is attracted to him he'll have no choice but to come around.