RE: What age is ok?
I agree with cashman,
testosterone is running wild at 15 it will give him a focus. however be his guide keep his feet on the ground don't let him gett carried away. just finishing my personal training exams everyone of them says not before sixteen but each individual is different. This is mainly because the bones are still growing apparently don't stop until you are 25, but if we all waited till then where would we be.
get started by just using excercises that use his own body weight. Definately try and get good protein from natural sources e.g. egg whites chicken breasts tuna blah blah we all know them.
watch him though mite start looking better than you ha ha