Hi B.,
As mentioned in earlier responses I agree that you should pay close attention to;
What kinds of food is he eating all day? I would assume not real good if its from the school. It is not always how much food he is eating but the types of food he is choosing to eat. And drink, soft drinks are huge and these kids spend alot more time drinking that than water. Encourage drinking alot more water. At his age I would not limit his food, nor sound discouraged about his weight or eating habits. Try to turn it into a positive thing and focus on the healthy habits side of things.
What kinds of activities (sports, chess club, etc) do you have him involved in physically and socially as an outlet for both the pysical stimulation and the mental /emotional stimulation. It is so crucial for a growing boy to be able to balance the quick growth spurts some go through, and with all going on in their heads about fitting in and making friends, etc.
What kinds of eating habits and social activities are the rest of the family (including mom & dad) involved in? This is important too, because as we grew up learning 'do as I say, not as I do' by our parents. It has now been proven that has no bearing on changing the mind of whether a teen decides to smoke, drink, eat excessively, etc. But, that they follow our lead by example. We are their role models in all aspects and they will more than likely follow in our footsteps unknowingly, because that is what they see and know. Not saying we should shoot to be perfect parents, its not possible but if your going to be teaching certain life principles to your kids, make sure your following them yourself.
How well do you or your husband communicate with him? Girls have a tendancy to want to talk, and have an easier time of getting out how they feel. Boys, (i have a 17 yr old myself) to get them to discuss how they are feeling or why they do certain things is like pulling teeth. But, keep it up.. the more you show that your interest in his health (not the lack thereof) the more he will begin to come around and slowly open up to you Communication is key.
Good luck hon.. raising kids.. aint it fun tho! It really is a challenging and very rewarding thing for us as parents go through. We learn just as much if not more about ourselves as we do about the little people we bring into this world. It is THE hardest job on the earth, and there is no training prior to, we just learn as we go. I would lastly recommend maybe to open things up, say you are going to start going for a 30 minute walk every afternoon and ask if he'd like to go so YOU dont have to do it alone. Then he thinks he is helping you out, and it will give you a chance to talk one on one plus get your excercise. Take care ~