Doctors say this is very normal for little boys and girls. You can approach the topic and ask where he learned it. But its most likely he found and it and hes never letting go. My son is 2 and is constantly down his pants.
Well, at the very least, don't tell him it's dirty. It's so normal for people to masturbate, the last thing they need is a boatload of guilt and shame started when they are 5.
My cousin had a problem with her daughter masturbating by rubbing her crotch on the arm of the sofa in the living room. She finally just told her daughter that she had to do this kind of activity in her own room, when she was alone. Oddly, that solved it, because the daughter wanted to be with the family more than she wanted to be by herself in her room.
If you think your child is being molested, by all means look into it. Otherwise, try to be unconcerned but to get the kid to use his own room and do it in private.