Some bad advice and some good in here. Lifting weights will not stunt your growth, I started at 12, and I'm 6'1". From a family where the average is in the mid 5' 5 range.
Light weights will not just tone muscle, nor will heavy weight just build muscle. Both will accomplish that, it's the actual intent of the exercise, whether functional stength or muscle building.
At his age, he should go for functional strength. Core exercises, body weight, and most important flexibility drills. And yes dearest, please go see a professional, you've been to the goddamn shows, you know most webmasters are in a shape, round.
