How much does he get to interact with his father? More than anything this is the age where most children get curious about boys and girls and the surface differences thereof and try looking at family members in the tub etc. They should not be sharing beds, they should be redirected and the first steps of sex ed should be started that you are going to build upon in the next few years.
um yah, is he being perved on? It is soo common.
sorry that you are dealing with this.
he could just be having normal stuff going on though too.
angi
Find out first why he is doing this the behavior could be a learned behavior, has he seen it in other children I think it would be a good idea from what you have said here to check out the dynamics in the family, does he vist anyone , is he alone with other people sitters, older family members ?