IMO Zita hit it in the head. He appears to have very little momentum to the plate with little leg involvement. In addition to that he does appear to be too deliberate which is potentially killing his momentum.
I agree that he could benefit from the Hershiser drill. It will help him get his hips going in the right direction, to the plate. It will also help him with his weight transfer getting his core more involved in the throwing motion.
However, the intent to throw hard must also be there. Speeding up his tempo will help him achieve the momentum shift he needs to get the legs and core involved. Therefore using the whole body to throw instead of throwing mostly arm.