Some of the attitudes on this thread strike me as alarmist, to say the least....Time to take a deep breath n' get some perspective. It is natural to for kids to explore their budding sexuallity in one way or another. Instead of viewing it as sex, it may be more helpful to see it as a foray into sensuality, ie, pursuits that bring physical pleasure, or gratifification. The enjoyment of food can be regarded in the same light. Children are inately innocent. you are projecting an adult agenda onto these boys exploits. They don't see it as u do. Sure, discuss it gently with the mother, n' the boys....but try to tread lightly. Since neither child is damaged by the experience, it would do no good to instill a sense of shame. I'm sure the little boy in question, means no harm to your son. Sex is every where in the media.....We become what we see!
I don't know the full details, but i think it would be significantly jumping the gun to suggest this child has been abused. We all develop at different rates. It's just as likely that this boy is more advanced than you are used to.
I know of many incidents where such behaviour is exhibited as a natural progression in our personal awareness and development.....I'm sure it is pretty 'normal'.