I completely understand what you're feeling, my daughter regularly hits me. There is so much I want to say, so much advice I feel I should give. But the reality is my advice is no good, I can't stop my daughter what can I say that would stop your son?
Having your child arrested is awful, the guilt, the shame you feel is unbelievable, but the sheer relief of having her out of the house for a few hours in un-measurable.
I guess the only thing that might be of any use to you is don't let it become normal. Do something now. I wish to god I had.