Ever watch the Grand Prize Game on the Bozo the clown show? Toss the ping pong balls into the three buckets, one near, one in the middle and one further away and win the game. This is like the "Goldilocks" theory- too small, too large, just right. Our brains and bodies learn "just right" by comparing it with too close, too far and adjusting the amount of effort required. Have your players throw from a shorter distance, then step it out to a longer distance. I go short, long, 46', but you can do a little trial and error to see what works best for you and yours. At every distance, he should be throwing as hard as he can.
This has worked for me- I'm interested to hear what else you people have tried.
Another thing I've done is put kids on a mound or a hill so they can feel the downhill movement. A natural tendency I've observed with 9's and 10's is keeping the weight above the pitching rubber rather than getting downhill. This exaggeration has helped my son feel that movement, and then we were able to work on how to create it from flat ground.