You can help your child to modify his food habits by asking him to keepa detailed record of daily food choices and activity levels. Study the food record together, and try to identify behaviours that cause weight problems. You then need to ask your child to identify a single change he is willing to make. Finally, one step at a time, you should be able to implement permanent changes in your child's behaviour which will result in life-long weight control. Parents need to practice what they preach by setting good dietary and exercise examples.
Treatment is a family affair apply the same rules to everyone in the house.
As a bonus, you may end up losing weight yourself.