A.,
This trick worked wonders for our youngest three children (ages 5,6 & 7), I just wish I would have known about it for my oldest 2 children!
After your son has mastered riding his bike with training wheels, do the following: Put seat in the lowest position. Take off both training wheels. Completely remove the pedals from the bike.
Let your son ride his newly "remodeled" bike around the yard much like the scooting toddler toys he used to ride. At first, he will probably not let his feet leave the ground, but that's ok. Soon he will get more comfortable and start to "glide" a bit. After he is gliding most of the time, and isn't putting his feet down as much, reattach the pedals. Remind him to pedal hard as he's starting out, and of course, remind him that you will be there to help him if he needs it, but I bet he won't.
This trick sounds "nutso", but we used this trick with our now 5-year old son last summer and he was riding without training wheels in less than a day. Good luck!