I am a boy, and being subjective to nudie movies and eventually pornography at an early age jumped the gun on my sexual activity early on, and I was sexually active from 14 on.
I would say do not encourage it, but do not allow it, and do not let the kid have a computer in his own room, and have a stern punishment prepared for when he dishonors the family or publc computer and gets caught.
Every boy will move in that direction, but it will benefit him to slow it down.
As long as possible. A more mature boy will handle/react to it better than an immature.
Plus kids are more impressionable when they are younger, and those impressions have long lasting effects.
imo