I hate to say this but, it's not a phase and you son has found a new toy!! I have a 2yr old so I ask a girlfriend of mine all the time about how boys behave because she has one the same age as mine, a 6yr old and a 8yr old-I consider her my boy expert!! lol She has told me stories of her oldest 'sword fighting' and 'look mom, I can make it stand up' comments!! Not what I wanted to hear either!! The wisdom she has passed on to me is at this age, teach him when and where is ok to touch it, and don't draw attention to him doing it unless it is in a place you don't want him to, like Walamrt while grocrey shopping!! hehe Sorry, trying to make light of the situation. My son goes through times of needing to have a hand down there or play with it during diaper changes. I've just started asking him when he is going to put the pee pee (when he is getting changed) in the potty when he does. Also, you have to remember, it feels good too when they play with it. They just don't know why it feels good!!
So don't fret too much over it, it's normal. Your son hasn't been molested or anything else obtrusive that I'm sure might be suggeted! Just either ignore him and see if the thrill stops or teach him when it is ok, like when he has to go to the bathroom, at home, etc.
Good luck!
S.