i too had this problem sometimes. The trick is that he is overthrusting for the length that he is. Just gotta remind him to concentrate and make sure he dosent over pull on the reverse thrust.

One thing that i used to do when i was starting out and it happend alot was if in missionary put my head down and watch myself as in went in and out. it just notice the different parts and make sure i kept the lengths right

Just my tip. hope it helps