It will be a dramatic change, even as a hybrid, so go carefully: lower tension, use the softer Lux strings (M2 Pro, ALU Fluoro) or other brands of softer polys (see my signature). Also, the cross should be a multifilament, perhaps even the Sensation he is currently using. My customers like Babolat Addiction in the crosses.
"I am easily satisfied by the very best." Sir Winston Churchill; ALLEZ ROGI!