If he is causing himself physical harm then I think you should check in with your son's physician for some advice on how to deal with his anxiety level.
how should i stop this habbit as he is injuring his skin
This sounds like a nervous habit-- a habit developed to deal with some level of anxiety.
If its not too severe to cause physical injury and/or its not hampering the child socially and/or it does not bother the child, then its probably just a habit --
If any of these other conditions exist, then the anxiety is a bit more serious, and you should take your child for a psychological evaluation to see if anything can be done to help with the anxiety. Also, if the biting is not the only evidence of nervous habits and/or not the only issue, then you should take your child for an evaluation.