Last week, after a very few false starts, my son took off riding without training wheels.
I had removed his training wheels and pedals about a month earlier, to get him used to balancing and steering. As he kicked around the blacktop faster and faster, I was pretty sure he was ready. On the big night, I ziptied a broomstick to the back of his bike. That allowed me to help him balance, without a potentially excruciating backache. He was riding more or less instantly, and I barely used the handle at all. Since he couldn't see me, I just didn't hold it, and trotted along behind him to build his confidence.
A few minutes later, I told him he was riding on his own (he checked with another adult just to be sure I wasn't kidding him) and off he went.
Next step - get a bike that doesn't make him look like he's training for the circus.