As others recommend, start by taking the pedals and training wheels off the bike and drop the seat low enough that he can easily reach the ground, and encourage him to take bigger and bigger steps.
This is what I keep hearing over and over. I was late to learn and so was DS. Luckily some friends got him a Razor scooter, and after he got the hang of balancing on that then he finally learned to ride without training wheels - around age 9...
He had done some OT earlier, and if he hadn't learned when he finally did, I would have brought him back to OT to learn to ride a bike. Sounds like the ebook is cheaper - do you have a link?
Thankfully DS14 has come a long way now - even skateboards a bit.
Smiles,
Grinity
