Orikoru
Tour Winner
Might sound silly but you might want to try a hypnotherapist, or at least a sports psychologist or someone of that ilk. Someone that can 'wipe the slate clean' mentally and give him a blank slate moving forward.It's just convincing him to use one that's the next battle. I think it'd do him wonders especially as he is a good putter and it's like you say just an extension of that really.
As to your question honestly don't know, I think perhaps to do with the fact over the years he's been a real student of the game that he's literally over done it tried to many different things and completely screwed himself mentally, tried changing to many times that now he's got no idea anymore what to do or how to do it just over complicates and freezes