In order for your son to maintain a healthy weight, creating healthy eating habits and increasing his level of physical activity are necessary. The emphasis should be on feeling healthy and strong -- not on weight. I discussed exercise in an earlier answer, so let us focus now on proper eating.
I suggest taking a family approach to eating healthy, so your child does not feel isolated as the one with a problem. Eliminate unhealthy food from your home rather than keeping it from him alone. Keep healthy snacks around the house and make them easily accessible to him. For example, keep cut vegetables in the refrigerator and fruit on the table for quick and easy snacking. Most importantly, set a good example. Children develop their eating habits from the people around them -- if they see you eating healthy, they will also be inclined to do so.
Feel free to consult with your child's doctor or registered dietician to help change your son's eating habits. Overall, do not discuss weight unless your child brings it up. The focus should be on feeling healthy and being strong! By praising his strengths, skills, and efforts, your son will always continue to feel good about himself.