This may not be universally popular, but there is always the idea of rewarding him in some manner for trying, for perservering even when it's hard/embarrasing/frustrating, and for succeeding. You could offer a new helmet, a new bike, a new cool biking t-shirt, a trip somewhere together, a new book on cycling, or really anything that might motivate him to just try it and stick with it. If it's really challenging, incremental rewards might be best, but you may just find that he picks it up quickly once he actually starts to try to ride more.
BTW: I agree with lowering the seat until it's easy for him to reach the ground. Anything, really, to make him feel safe and in control.
