I have a rod in my left leg, which isn't my good leg btw and my brother also has a rod in his left leg,he broke it twice, mine was once. Your Drs have to advise you health and safety procedures it's part of their job. Your son's metal rod is not going to bend,please don't go telling others that,my physio assured me of this and he was one of this rare specialists in England, NHS specialist I may add. Your sons leg is sturdier then your leg with the rod in it, if he breaks it again, it's not going to bend it's just going to be broken and painful but his body will let him know that. My leg went like jelly when I broke mine. And whilst I'm only just recovering and after six months able to go to the gym,my brother ten years on is an amazing rower and he walks every where, plays squash, and probably other sports I don't know about,he broke his leg playing rugby and would still probably go back if his work wasn't so demanding. My advice to you is let your son live life and suffer the consequences, thats what being an adult is. If he's sustained a motorcycle accident I understand that your worried but jumping itch all the safety procedures is hardly going to do worse. Sorry to sound harsh but since breaking my leg I'm a tough love kind of girl. My leg won't be the same and neither will your sons, but I know someone who runs marathons now and she broke her leg. If it's something your son wants to do you could do that with the him and prepare with all necessary procedures.
edited to make it clear this is a not a quote of the original post
