From a guys point of view. They are very right! Ur sons masterbation habits are totally normal. I have 3 sons, twin 3 yea olds and a 6 year old also raising my 13 year old nephew. They all have been masturbating since they were about 1. It won't hurt ur son, it deffinently won't hurt his growth or his hormones. If it did most men would be really short and hairless! Lol also, don't freak out if u find out he's using porn or anything. Most of the time they are! Or they are experimenting with their male friends. If u catch him doing that don't freak. Experimenting is normal aswell. The only time u freak out is if he's doing it with a girl. Maybe u should sit down and talk with him or have a grown male who is close to him talk to him about what's going on with his body, masturbation, and sex. Most boys don't want to talk to their moms because its embarassing, so don't take that personal if he didn't. Don't stress this though. He is a growing boy and as long as he is doing it in private, don't get worked up about it.
11 is NOT early for masturbation...for some children it actually starts in the womb, believe it or not. I agree with everything amberette said...and I want to add that he will NOT lose hormones through the act of masturbation. if anything he will stimulate them. it will not affect his growth unless he's doing it so much he's not eating! so don't worry, just keep an eye out for indicators that he's becoming sexually active and let him have his privacy beyond that. it's completely normal for him to do in private.
Masterbation is completely normal in his age group. As long as he is doing it in private, and he isn't trying to become sexually active at his young age, it is fine. If it becomes a huge issue, and he is showing signs of wanting to become sexually active with others, or other issues like interuption in mood....then maybe it should be discussed. Most boys masterbate. It shouldn't cause any issues with hormones or growth.