I don't think it's necessarily grass. If it were a choice between the 4 slams, then yes, Wimbledon would be the one he's been underperforming at the most. But if there were more tournies on grass, I don't think he would overall perform the worst there. For example, I think Fed has better chances against Nadal on grass, than on slow HC, for example.
I think it's close between grass and slow HC.
RF (4-1-7-5-6-22-80), RN (1-9-2-2-0-27-64), ND (4-0-2-1-3-19-45), AM (0-0-1-1-0-9-28)