He plays with some iteration of the pro staff 6.1 18x20, probably older versions like HPS or so.
This guys tend to like things the same way once they find the right racket, and it's not rare they play with the same racket throughout their career. They tend to try minor changes with weight and balance, and sometimes they change strings, but not much
He probably plays with a pro stock version of that racket. All retail 6.1's have around 70 RA, that's too much for a pro player, specially a guy with wrist problems like him. I've seen pro stocks 6.1's on string forum with lower RA, on mid 60's range, so probably Wilson gives him older 6.1's version, with softer layup like those