This is one scenario where the three point or forward seat is very useful. If he is light in the saddle then he must put weight in his heels or he will fall off.
Have him really focus on bringing his legs back, with heels down and keep no pressure on the seat of the saddle. Having his hands on the neck for a "just in case", but with no pressure is a good idea. Have him also focus on sitting up straight, and not just tipping onto the neck for support.
Gradually as he masters a light seat in all three gaits, have him start to sit down in the deep seat and focus on having the same feeling of heavy heels. If he loses it, get him to take his legs off the horse and re position them, or go back into the light seat.