He doesn't go down far enough therefore does not get enough leg power which causes his arms to throw instead of guide. He needs to get power from his legs and his shot will be more fluent.
I don't necessarily know that this is the case. I have no idea how many free throws he shoots, but it is probably mental for the most part...or something goofier like the yips. Either way, he has no shot at playing at the next level unless he fixes it.