I second the balance bike idea. We have a trail-a-bike and although it increases confidence, it does zero to teach balance.
Take a 16" or 20" bike and take off the training wheels, pedals, cranks, and chain. Set the seat to a height that he can stand over with a few inches to spare. Start on flat ground and let him push himself around with his feet. He will quickly figure out how to steer to keep the bike under him. This method works so much better than training wheels, which only teach them how to pedal, not how to steer or balance.