Sometimes it helps to let them practice riding on the grass in the backyard. It's slightly slower and requires slightly more strength to push, but the slower speed allows them to concentrate a bit more on the movements and, if a fall happens, it doesn't hurt. Also, backyards tend to be more private.
Another tip would be to hold on the the back of his shirt or jacket rather than the back of the bike. This allows him to balance himself more -- and to feel what that feels like -- but is at a better height for you so you're not braking your back running along behind him bent over towards the seat.
Also, a flat, straight, private bike trail is a good bet. And, if he's worried about falling or stopping, then long pants and knee and wrist pads can help alleviate some of that fear.
Good luck!
