Get a Hula Hoop or even a pole. Rid with him in tendem using the hula hoop or pole for him to hang on to. This gives him the extra balance he needs but gives him the feeling that he is doing the actual riding. I have found this to be very helpful and you can play fun games this way.
Do you ride behind him (i.e. up hill) or beside him? I'm goofy and I think he'll be regular based on the foot he uses to push his scooter so if I was beside him I could actually face him which could help.