you can have him try throwing a back up ball slowly. Stand behind the foul line, stand like he's posting up (to clear the hips), and slowly do a 1/4 swing back and forth and release a back up ball after the third or fourth swing.
I say slowly for several reason.
1. Not everyone can throw a back up ball, doing it slow will prevent injury.
2. Doing it slowly also allows him time to get used to the feel of the release, but the key is being aware of the hand position throughout the swing.
3. We all know everyone have different levels of learning a new skill. Some are quick and some just need more time. He seems he needs time.
Once he get used to this drill, increase his swing another by 1/4 and focus on his swing and and hand position.
Performing a movement while being aware of our body is important. Every move has a meaning and a purpose. If one component (hip rotation) in throwing a punch, for example, is ignored, the whole movement is wasted - lack of effectiveness, power, and/or speed.
Right now, it seems he's not aware of his hand position.