If there's one problem that can absolutely lock a player up, its a fear of the ball. The odds of getting hit are probably around 5%, but that fearful story being played in the mind will sabotage the other 95% when they were safe all along.
The fear overcome's many players, to the point where them stepping out can feel involuntary. And we've all tried throwing tennis balls but for me, that has never worked (throwing tennis balls at kids IS fun though!). But there are REAL answers for someone dealing with a real fear, which is what we're dealing with here.
While there is no "Instant Magic Bullet", I truly believe these methods are the closest thing to a true answer as there is.
So give this a quick listen, and if you are looking for the best hitting program available for youth players (put together by a 35 year MLB vet and current hitting coach), then check out this link here
Good vibes and praying for Courage for your player!