Well, his movement is not very good, and it can get slippery on grass. Plus his return doesn't work as well on grass. Not that either of those are great anyway, but they become even worse on grass. Also, he's tall so if anyone slices to him and the ball stays low that causes automatic problems.
It seems backwards, but clay is a better surface for him because of a higher bounce. Also he gets more time to set his feet and his strokes because of the slower nature of clay as opposed to grass. And he can slide on clay if need be, but you can't do that as well on grass.