The experts say 12 but some light weight lifting won't hurt. It doesn't stunt your growth. That's a myth. He won't gain muscle because he isn't producing testosterone yet, but he will get stronger. Total supervision is needed if he does to prevent injuries. Emphesize good form. Explosive movements build strength. Focus on bench, squats and deadlifts. Make sure he's getting enough protein (anything from an animal that's not fat or bone).He needs that anyway for his brain development. Your brain makes new synapsis pathways untill your 10, then starts to get rid of the unused ones. So the more he makes untill he's ten the better and protein feeds the brain. If he's just trying to gain some weight increase good calories. Low fat meats and carbs. Some good fat like omega-3's are great and important for all development and strength. Children's multi-vitamins are also very important.
