My own worst enemy?

Is Tiernan his own worst enemy? I'm having a hard time dealing with his self-inflicted injuries... mostly because I can't understand why he does it. What does he get out of banging his head on the floor or biting his hands or hitting himself in the head? And more importantly, how do I get him to stop? It's one thing when we are home and I can distract him or physically restrain him, but when we are in the car and I can't reach him there is nothing I can do. We have resorted to wrapping his hands in a gauzy material so at least he has some padding on his hands and can't do as much damage. But isn't there a better way???

~Mom