Honey,
There's nothing wrong with what he did. He is exploring his body. He probably "tugged" it, it felt good, so he continued. It's natural and okay. So don't panic!! You might give him a complex if you freak out on him and make him think it's wrong to explore yourself thus making him ashamed of his body, and that could be an effect for years to come.
It could be behavior he has seen so be observant who he is with,it is also Normal to be curious about his body, children are great copiers, so check out the gut feeling you have just in case.