he'll grow out of it. it is probably just a comfort thing like thumb sucking. i used to rub my hands really fast and my mum used to nag me all the time but it didnt stop me. i grew out of it. it wont do him any harm.
you might be able to stop him by giving him a small toy or a small teddy bear to chew/ suck on if he/she is nervouse or, you could help him through the nervs.