But here's what I think, assuming it is not an arm problem but rather just in his head. I think if you try too hard to "hit the target" right on the button, you can sort of aim the ball instead of just throwing. You end up pulling the string a bit on each pitch, which can really throw you off. I think it is more of a problem for guys who are not power pitchers, who know that location is really important for them, and who therefore try really hard to hit their target. The further off you get with each pitch, the harder you try to "aim" for the target you are trying to hit, and it just gets worse as you become a head case.
If it were me, I would experiment as follows. Instead of trying harder to hit the target, in my mind I would try to throw "through" the catcher. In other words, instead of trying to hit a particular spot, I would imagine that the catcher is really two feet beyond where he really is, and try throwing through where he really is to reach a destination two feet beyond where he really is. I think that might keep me from pulling the string on the pitch to try and hit a particular spot.