My grandson was a natural and began to ride on his own, with Mom or Dad always supervising nearby, somewhere between 2-3 years old. He was pretty amazing.

You may want to think about, first, one of those child seats that attach to your bike, then when your godson is a little older, a trailer bike. That way, you and he can share the experience of riding together even before he gets to ride on his own.

By the way, I have heard that children should not be placed in any kind of attached cycling device--a trailer, seat, whatever--until at least ~18 months of age because their neck or spine hasn't yet developed enough strength to hold their heads steady enough.