L.S.
I'm not sure if this will work but how I taught my daughter to ride in one weekend:
1)Find a gently sloping grassy hill (fortunately we have one in our front yard) without obstacles (no trees / fences for a wide stretch).
2)Lower the seat of the bike all the way down so his knees are slightly bent and his feet firmly on the ground (no training wheels - for starters you could leave them on then once he's gotten the confidence of the lower seat and feet on the ground remove them).
3) Have him sit on the seat and use his feet on the ground to move the bike around on flat (grassy)surface first (it is easier to fall on grass).
4)Have him move the bike down the gentle slope the same way.
5) Have him practice lifting his feet (one or both at a time quick and low then longer and higher up to pedal level) while in motion ("glide"). Have him "glide" for longer and longer periods (on the way down the grassy slope). Then progress toward putting feet on pedals.
6)Add some pedalling down the gentle grassy slope and lift the seat gradually. You might want to go thru this as you raise the seat a little bit every day until he has the feel of his balance point at the higher seat level.