Are you over restricting his activities b/c of the ITP? He may be frustrated and angry at the loss of some of the activities he used to do. If he was in daycare and now is not, you'll need to find other ways to direct his energy. Walks with you, playing in the yard (if you have one), sandbox (kids love that, little chance of injury), finger painting activities...keep him busy and occupied.
Just a thought that the sudden change in his routine might be frustrating him.
You have very little choice about how to manage this, and the fact that your son has ITP really does not influence very much the behavior management decision. At the first, even little, sign of aggression, place him in time out for several mintes (e.g., 5 minutes). Start the time only when he seated in the time out area (a chair, the bottom step of a staircase, e.g.) and he is quiet. Track the time with a portable digital cooking timer. When the time expires, ask him: 'How come you're in time out?' When he answers sometnig to the effect: 'Because I hit....', tell him 'No hittting (kicking, pushing, etc.).' It's OK to use this method when he acts aggressively toward himself as well, because you are trying to extinguish the biting behavior. If he did not have ITP, I wouldn't suggest timing him out for biting himself, but you're trying to avoid situations that involve possible bleeding.