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.
"When a man is tired of London, he is tired of life"...Samuel Johnson 1777
"When a man is tired of Federer, he is tired of tennis"...stringertom 2014