Toddlers and masturbation

I know the title may seem inappropriate but this is what I can think of right now.

I delivered baby #2 June 18th, and every since bringing baby home. I have noticed my two year old is very facinated with playing with his private. I catch him with his hands on his "wee wee" and sometimes legs in mid air with his finger near his anus.

I don't believe my son is being molested as he is with his father and I all the time.

I need to know if this is normal and how long would this last?