Buy low calorie food. Eating fewer calories is vital in a weight loss diet. Drinks with no calories, such as water or plain green tea, can fill one up and decrease appetite. Eating carrots, celery, or iceberg lettuce (without any dressing or dips) is nutritious and also has very few calories. Avoid fats because fats have a lot more calories than protein or carbs. Whole grains are better than sugars & simple starches.
Motivation. Set a goal for the weight loss and make it fun. Give prizes when he reaches a certain weight (be creative, I am sure you can motivate him!). Cook healthy for him, look into the glycemic index and take out the bad carbs.