If he's got an extreme (grip or grips), try to keep the ball low and slicing. Step in a chip it. Generally, figure out what he doesn't hit as well and feed him a steady diet of those type of balls. You might also try big, looping topspin, as deep as possible. The key is deep and looping to keep him behind the baseline. That way, he can't hurt you with his big topspin shots.
Men's and Women's Head Tennis Coach, Friends University
Dunlop f2.0 Tour, Lux 4G, 4.5 all court, USRSA MRT