The first time I walked in my son's bedroom, he was playing with my friends niece (he was 4 and my friends niece was 2), and he had her pinned down on the bed and kneeling on her arms. She was screaming. The second time was with my friend (his dad's partner), and he asked her to bend over on the bed so he could kiss her neck and feel her boobs. We have caught him on a few occasions doing inappropriate things, but only to girls, and when no one is looking he tries something. He is 6 now, and we have a little girl who is 18 months, I was running a bath and she was on the bed naked, the little boy ran up the stairs, got in bed, and when I come back up stairs he had a guilty look on his face as he was trying to pull he's pants back up. What do you think I should do? I'm wondering whether or not to phone social services.
You should definitely seek professional help for this child, such as counseling. This is not normal behavior for a child this age.
Getting counseling serves two purposes: protecting the other children he is around, and determining where this behavior is coming from. As a foster parent, we typically see small children acting overtly sexual when that child has been abused themselves, or exposed to adult materials (pornographic videos or magazines, etc...)
A couple of similar questions:
Chart showing normal and abnormal behaviors in small children: