Why does he NEED to learn to ride a bike right now? I would suggest just leaving the bike and letting him try it when he's ready, maybe when all his friends are riding and he can't keep up on the scooter he'll be more encouraged.
I didn't trust my dad and my older sister for some reason when they tried to teach me to ride, but a few weeks later I was at a friends house and they taught me on one of their bikes. For some reason it was less scary, I may just not have been ready yet when my parents tried. Everyone is ready to take on different challenges at different times. Let him wait til he's ready, even if he never learns to ride a bike it won't be the death of him.