Ideally it's preferable to wait until he's around 16, but 14 at least. Muscle responds okay to it when people are young, but bone is not so forgiving, unless you go very light and probably never exceed around 75% of a muscle groups tolerance, whatever that would be.
Too much could affect the way bones continue to progress and as skeletal finishing does not occur until 17, there could be potential affecting of that process before completion, if someone was loadbearing to maximal levels or not far off them.
I would suggest he may be able to attempt some kind of bdoeyweighted structure, but with absolutely no added resistance at all, not even a couple of books or very light weight.
This might be sonething for him to do.
2 sets Squats, 10 reps.
2 sets Lunges, 10 reps.
2 sets Good Mornings, 10 reps.
2 sets Vertical Pec Flyes, 10 reps.
2 sets Bent Over Rows, 10 reps.
2 sets Shrugs, 10 reps.
2 sets Leg Raises, 10 reps.
All these are using absolutely 0 resistance, but are using limb weight only, but he needs to be properly warmed up first, so I'd give it 5 minutes at least, of some stretches like scissor and pulls, windmills, hip rotations, knee raises, etc and perhaps a bit of jogging on the spot for 2-3 minutes beforehand as well. Get a bit of heat going and circumlation, before doing the dynamic stretching.
I would supervise this though, don't let him potter about doing this whilst you're training, so you know he's doing it right and can monitor how he feels, ot make sure he avoids issues and if he experiences any you can be aware of them and assess if he's okay to continue or may need a few days off.