I am 28 and try my best 3 times a week to exercise but haven't had much luck loosing the weight I have gained. I have also tried eating lighter, healthier foods. Motivating another person to lose weight is a tough problem. All you can do is lead a horse to water if you know what I mean. Encourage him perhaps by offering to do some of your activities with him.
Company is always more fun that doing tedious things alone.
Perhaps a tennis game or racquetball is more inviting than using a workout machine. If there is an eating disorder however, things are more serious and there should be fewer options. When a person is ill, he needs someone to help make decisions for and with him. Particularly in individuals who struggle with other addictions as well as with food, the support and structure and communality of OA can be invaluable. Most treatment for eating disorders involves cognitive-behavioral focus with a psychodynamic overlay within the therapy process, but there is something wonderful about groups, supportive and friendly and motivating.