As Sam says the first step is to ask for a mandatory reconsideration of the decision. It's hard to know from your post why the DWP have stopped PIP. Was there a reassessment? (Did your son have to fill in a form and go to a medical)? Or was PIP stopped for some other reason? If you have the decision letter about PIP, please post back and tell us what it said.
You should ask for a mandatory reconsideration within one month of the decision, if you can.
Do post back if you need more information.