Hi, I'm sorry that you're in a tough spot, but it doesn't sound like your husband is looking for your support. It sounds more like he's putting you through emotional blackmail and holding you hostige with threats of increasing his eating and making his health worse.
Here's what I would do, and I would not do this unless I were prepared to act. By prepared, I mean be prepared to move out and live somewhere else. If you can't afford that, speak to some of your family members and see if they will accept rent for letting you and your son stay in case you need to move out.
Honestly, if your marriage and your husband's health cannot be saved, you're better of on your own as a single mom than locked into an unhappy marriage. Remember your son will learn from you what is acceptable beghavior in a marraige. If you put up with being held hostage by your husband's self-destructive behavior, your son will think this is OK and normal and won't have the knowledge on how to get out of a bad relationship or maybe treat his future spouse the way your hubs treats you.
I would be honest with your hubs, but after revealing your feelings let him know you're willing to do whatever it takes to help the marriage and his health improve- but there is a condition to this:
he has to be an active participant in that too. This solution should involve marriage counseling, it sounds like there's a lot of issues here that the two of you may not know how to work out alone.
1) If he refuses, move out and get the wheels turning towards a divorce.
2) If he threatens you (ex: "if you talk to me like that I'm just going to eat more") that is the same as a refusal, move out and get the wheels turning towards a divorce.
3) If he agrees, hold him to it and make it clear that if he slips back into old habits you will take that as him neneiging on the agreement and you'll have to separate from him. Sometimes people will agree just to prevent you from walking out the door, but they have no intention of following through past an initial token gesture. Be mindful and prepared for that.
Don't get roped in with guilt or threats. He is going to do what he wants and people don't change. The only time people are motivated to change for the better is if they want to do so for themselves. But there's a chance he might come around and realize he's ruining his health and his family and then make positive steps to change that.
So hope for the best, but prepare for the worst.
Best of luck, I hope it all works out.