Work one thing at a time. forget knee bend, forget lowerbody, forget hip rotation, i would not even give the ball to him initially. just hold it in trophy position, swing up and stop. the racquet face while dropping, you can tell him should be parallel to the his head/back. this way he CANNOT get a waiters tray position. break down each of the milestone positions. work on it and slowly build on each one. i would say for a smooth flow of basic full service it should take 1-2 years. this is just basic. then as he grows and gains strenght then add on more pro stuff.....
Also the key is to give him a 25inch racquet to begin with if not 23 inch.