Please stop worrying. Here's some accurate information for you:
People self-pleasure from birth to death. Our self-sexuality is an important part of who we are. There's never been a shred of evidence that self-pleasuring is in any way harmful, at any age. In fact, there IS evidence that self-pleasuring is an important part of how we learn about our sexual responses. After all, how can tell a partner how you like to be touched if you don't find out for yourself?
For more information about childrens' and teens' sexuality, I highly recommend the book, "The Family Book about Sex," by Dr. Mary Calderone. Dr. J
what do you mean about penetration...sexual intercourse? If so then yes it is bad for children. Now if you are talking about masturbation...it is normal for teenagers going through puberty to masturbate and explore their new found sexuality. I personally would not condemn him for it. I have raised 3 boys (21,18,15) and 2 girls (20,12) and I know my boys were playing with themselves and I did not interfere with their exploration of their own body. Just think about it his body is changing and he is getting all these new feeling and the opposite sex is starting to look different (or in some cases the same sex, gay teenagers go through the same thing). So to sum it up I don't think it is bad for teenagers to "Play" (I say explore) themselves as long as it is in an appropriate manner ( in private, because it is a private matter).