concerned parent of child who explores others

when my son was 3 I was babysitting for a friend and caught the little boys, my 3 yr old and the other 3 yr old boy, trying to "change" each others diapers. The other boy was in diapers and my son was not. I stopped babysitting for the other kids when I caught the other boy over the top of my son grabbing his privates, I immediatly took my son to out local doctor and she sent him on to counceling, he has 4 sessions and she said he was fine. Now after a year or so since the incident we have had to more incidents, once we caught him with his pants down and 2 others girls, and the last one he told one girl to take down their pants and he was taking his down too. I have talked to him about the "good" and "bad" touching and how he only should let doctors or mom/dad look at his privates. And stressed how important it is to not show or touch any one else's privates. Is this normal for children?  Is there something wrong with my child? What can I do to help him and make him understand better?