Is there any possibility of using a two wheeled bike with your son? The occupational therapists at my local hospital set up a programme to teach cycling to children who had been medically referred for co-ordination problems. I worked with them as a coach, and was surprised to see that even kids who initially had problems walking and pushing the bikes were able to cycle successfully after a few weeks. We just used the saddle down, pedals off technique to turn the bikes into balance bikes, and gradually progressed from walking, through paddling with the feet, scooting and gliding to full pedalling.
I don’t know what your son’s situation is, so this may not be appropriate for him, of course. I was impressed with how well our kids did, however, even in cases where they continued to score low on conventional measures of motor skills, so it might be worth a go.