When my grandson was about the same age, my daughter found a bicycle that didn't have pedals. It was a two wheeler so he leaned to balance just like on a regular bike but he didn't have to contend with learning balance and learning to use the pedals at the same time. I'm not sure where she got this bicycle from but it was great. He could really move quickly, pushing with his feet. Something like this might help your son. I wish I could give you more details about it.
I think I heard someplace else years back about these. Hum...
