Aiden saw a kid around his age riding a bike without training wheels this summer and it renewed his interest. We went to the church parking lot and after he rode his scooter around for a few minutes, he wanted the training wheels off. I held the back of his seat to keep him upright and we rode down a ways and back.
After riding around like that I took my hand off the seat but then still ran behind him with my arms out ready to catch him if he tipped. The more he practiced, the farther away I got until he was riding on his own. I just had to help him start.
Once he was a pro at riding, I showed him how to keep one foot down and petal with the other foot to start. It was hard for him but he found that if he instead pushed off with the foot that was down a few times, he could get going. From then on, he's wanted to ride his bike every day. He's getting better and better.
I put some clips that Andrea recorded together. It's worth sitting through... or at least skipping to 2:22. Your abs will thank me.