What you say is logical and reasonable, but my brother is 86 years old and still skinny. He doesn't eat as much as he used to, but an awful lot for someone his age. Yes, the subject of this discussion should be checked out.
Metabolism, tape worm, illness, disorders... the list goes on and on as to what it could be. Talk to his doctor about it. If he constantly eats, it is NOT healthy, and it WILL catch up with him one day.
One of my brothers was just like that. And not just during puberty either. Doctors never found anything wrong with him and he has always been in excellent health.
He could be going through a growth spurt. Has he put on much height lately?
He could just have a metabolism that is fast - his motor is always running and he burns up the carbs.
It could be ADHD or ADHD medication related - is that why you posted on this forum?