I have read and answered this post before, possibly on another site, as the last post says,, its possible he has b een taught to do this it could be learned behavior so check who he is with alone keep an eye out, and when he does it distract him from it, take him away from the Chair or whatever ,and say No! firmly and then read to him or put some Music on, do something else to take his mind off it.
I have never heard of this, this young. I know they will start being curious about their body around this age, but this is different. I would just evaluate who your child is around. Does he say anything when he is doing this? Have you asked him why he does it?