My 2.5 year old son has always been fascinated by his penis but I thought it was because he didn't get to see it much (except in the bath or when being changed). However he's been potty trained for a couple of weeks now and some days he will play with himself CONSTANTLY. If we tell him to stop, he gets very angry and tells us to leave him alone and flat out refuses to stop.
Today I picked him up from nursery and they said he'd been doing it all day and wouldn't stop. I asked if that was normal for little boys and was met with a firm NO. 
I have tried talking to him about it many many times and he simply does not care what anyone says when it comes to this specific issue. He's always been a bit ummm "wilful", but generally when told no, he will stop whatever he's doing.
I don't know what else to do. Can anyone offer advice? Also is this really THAT abnormal? 