My son has been doing this since before he was even 1 and he will be 6 soon.
Doctors never said anything and therapist neither.
I just let it go.
He used to do it to anything.
But now it's pretty much when he wants to nap or sleep. I suppose it's calming.
I've only seen him once or twice just recently doing it when watching tv by himself.
Since, no one has said anything after I've mentioned it, I figured I was just part of his ASD and sensory so I let go.

It could be sensory related. you should speak to an OT that specialises in sensory issues or an behavioral therapist.. It could just be hormonal as well - boys being boys. Even speak to his doctor. children on the spectrum experiences hormonal changes more intensely than neuro-typical children - either overactive or under-aroused.

My son is 3 an does this all the time he has sensory issues the doctor's say it's normal so I just say it apart of my son an I keep reminding my son that we do this in your room only!!!!

My son has just turned 4 he started when he was 2 I didn't think anything of it but I caught him tonight in his bedroom and he was soaking wet I changed him and saw his tail was erect and really sore I don't no way to do about it either

My son has done the same from a very young age. Stopped for a awhile and now that he’s 14, he rubs pretty vigorously into the floor and has started wearing holes in the knees of his pants; always on the left pant leg only (go figure). Good news is school hasn’t brought it up as a problem and it seems to be confined to when he’s in his bedroom. I’ve gone into his room to see what is causing all the racket and he pushes me out the door and closes it again. At least he seems to get that it is a “private” behavior. I have found that not making a big deal out it when it started when he was younger was the right way to handle it. Still is.