Thanks for the replies. I had not considered balance bikes because in my mind I was thinking they were for kids under 3.
I think I will try taking the training wheels and the pedals off of my son's bike and teaching him to push and balance on it and go from there. Maybe the size of the bike is too constrictive now for him to comfortably pedal and we're just adding an obstacle.
Thanks for the link for the Islabikes website and sizing info. I did not know there were quality mountain bikes available for small kids but that is a good thing to keep in mind.