I understand that your husband has issues of his own and that this is distressing to him, hence the ranting. That doesn't make it any less abusive. If he can't see it as such, maybe you can get him to understand that shaming and coercing are going to backfire and create a low level of self-esteem and an unheathy eating disorder, whether it be obsessively eating too little or too much (with a boy, most likely the latter.) The fact is that your son probably normally doesn't eat more than the people around him and his body just deals with the food differently. This is genetic and is certainly nothing he should be yelled at for. It's not his fault. If your husband continues to behave as if it is fault, though, your son will develop a complex about it, develop an emotionally unhealthy relationship to food. He's going to end up hiding his food consumption, binging, and not paying attention to what his body really needs. It's probably already started, thanks to your husband.
The only hope there is of your son achieving a healthy weight for his body (which, it must be said, may NOT be what your husband perceives as "healthy") is to make it a non-issue so that the psychological element is not undermining his body's ability to self-regulate. That means apologizing for past ranting, and saying no more about it. Of course it would also be helpful to provide appealing healthy foods and offer opportunities to be physical in a fun way that isn't for the express purpose of losing weight, i.e., something you would do even if the weight wasn't an issue. Until your family is living this way naturally (and not just as a tactic to get your son to change) it's not going to feel natural for him and he's not going to do it.
I know your husband doesn't know any better, and I hope he's acting like a jackass only out of concern for your son and not because he is ashamed of having a son that isn't what society regards as perfect. I'd like to give him the benefit of the doubt and think he could get past it. If he can't, you need to intervene and start speaking up for the boy. Tell him that his dad is wrong, and talk to him about how some bodies process food differently, that bodies grow at different rates, about how some foods are addictive and some are harder on the body than others, that it isn't his fault and that there is nothing wrong with him. That's the best thing you can do for him.