Originally Posted by mynameisjim

I think people missed the point that he would be working out on a bowflex. It would very hard, if not impossible for a child to harm himself working out on a machine. Fact is, kids just aren't strong enough to put enough stress on their joints to cause problems.

That being said, you don't want to use pro level training techniques like forced reps or anything like that. Compound movement (involving multiple joints) exercises like bench presses would be best.

Very high reps are actually more dangerous because he can develop overuse injuries if his form is not proper. 15 or so reps per exercise should be sufficient so adjust the weight so the 13th, 14th and 15th are difficult, but not impossible or causing extreme strain. Make sure he takes days off, don't train the same body part more than once a week.

Key here is supervision and making sure he doesn't get carried away but instead focuses on form and technique. He won't build much muscle but he will get stronger and tone up.

I think the upside far outweighs the very small risk. If he gets into a good routine he may adopt a healthy lifestyle that he will keep the rest of his life which will pay off huge dividends for decades to come.