Be vigilant about any drastic weight loss attempts. Attended my 50-year high school reunion. The lightweight guys that had plastic suits, spit cups and starvation diets all look horrible. They look twenty years older than they are, bent over, joint pain etc. beyond a typical 68 year old. Let your son eat, grow and burn energy. When needed, 3-5 pounds over time can be accomplished safely. He has a long life ahead of him, good luck.
Just an old fart observation.