This was my problem... Has he been tested for Insulin Resistance?
I'm 17... I had WLS (Vsg) a year and a half ago. I've struggled with Insulin Resistance since i was 3 years old... It was determined it was entirely genetic, because the way my diet and activity level was, it just didn't make sense that i brought it upon myself.
If he doesn't, maybe see a dietitian? Ask them what you can do.
Could he be starting another growth spurt? That could cause the hunger.
I know my parents had to put locks on the cubbords so I wouldn't get into them as a kid because i was always hungry because of the high insulin levels.
I know juices were cut out, only skim milk (nothing higher than that, no 1%, 2%, whole, etc..) wheat instead of white bread, etc.. You just have to plan out meals and serving sizes.. Let him have as much of the fruits/vegetables as he wants. But other things you'd need to limit.
(just speaking from what dietitians had my parents do for me.. had they not done it I'd probably be a hell of a lot worse off now, than i am.)
If your doctor can't figure it out and he continues to gain weight, get a referral to an endocrinologist. No one figured out what was wrong with me until i was 7 or 8 years old, by which point i was already extremely overweight despite all the pre-cautions everyone took and my pediatritain constantly saying i was fine.
I really hope you figure out what's going on. It may be nothing, but on the small chance there is something metabolicly going on, you don't want to miss it. xx