First of all I agree with Ulter,
His epiphyseal plates or bone plates in his body are still growing and have not healed or formed hard articulating cartilage.
This is through out his body not just legs and arms. Each long bone has at least two growth plates: one at each end. The growth plate determines the future length and shape of the mature bone. But the damage will be seen everywhere.
When you take gear the plates close prematurely, and not with solid bone.
Your long bones don’t grow longer your height is stunted and your bone plates are still not solid bone, so they damage much easier then if left alone.
I have been in over 5000 surgeries in the past 30 years, and have seen kids on gear having arthroscopies whose articulating cartilage looked like cracked eggs, because of gear. When you remove the articulating cartilage, you next injury you will need a total joint, not nice at 19 or 20.