My son was riding his push bike on the footpath he crossed a driveway and collided with a car who was driving out his head dented the car and his bike left a scratch on it Who's at fault?

The driver of the car is. Footpaths are public domain, whether they cross a driveway or not. Your son had every right to be there. It is the responsibility of the car driver to watch out for other cars, pedestrians, and bikes.