At his age his hormones are still doing what they’re going to do and he’ll naturally gain weight as he grows. However, instead of steering him towards supplements and liquid calories, steer him toward whole foods. A kid doesn’t need to worry about a six-pack, and at his age he’ll almost never get this kind of growth again.
Have him eating, eating, eating. Not to get sick, but when he’s hungry, eat. A good protein source, a good carbohydrate source, and a good fat source in every meal and he’ll be fine.
If you’re going to steer him towards anything, steer him towards researching everything he’s curious about, gaining weight safely and effectively being one of them.