For drills have him try "Discus Bowling". Have him take discus and roll it across the gym floor or grass in field with the goal of getting it to go as far and as straight as possible. This will help teach the feel of how a discus is supposed to come off the forefinger. Also have him throw the discus vertically. Only trying to throw the discus about a foot or two in the air at first. As he gets the hand of this have him increase the heigth slowly. These drills should help him develop a feel for how the pressure and pull should feel coming off the discus correctly.
Another thing to watch for is if he is pulling his throwing arm a head of himself. This will cause the discus to flip through the air. If his hand is pulling before his hips come through at the finish the discus will not get proper rotation.
Good luck with your throwing endevors.
Sincerely,
The Staff at Athletic Points
http://www.athleticpoints.com