I was 6'3" at 17 and was at 170. I was dunking a basketball two hands behind my head. I was not skinny, but I was not strong either. I was not unhealthy, and actually needed to lose fat from my body then. Even growing children can still use the BMI chart as something to use as a general guideline. Without seeing pictures of him, I would hate to guess, but no, I really don't think his weight is anywhere near an unhealthy stage. Especially since he seems to be very active.