Motivated by the neighbors boys, the second of whom mastered training-wheel-less riding on Friday, Sam and I spent some time Saturday afternoon taking off his training wheels and learning to ride on two wheels. All it took was about 5 or 6 runs up and down the sidewalk a block or so with me holding on, and he was good to go! From 4 wheels to 2 wheels in about 15 minutes - much faster than I feared it might take!
(Of course we had to take Sadie's training wheels off as well - and she didn't do too badly herself. She might make it before fall is over too!)
Here are a couple videos. You'll have to turn your head sideways - I forgot to tell Joe to hold the camera horizontally when filming and I don't know how to rotate the videos once they are on my computer.