Have him start in a position like a big "L", with his hitting arm being the bend and toss arm the long part of L. Basically just before trophy pose, make sure racket is on edge, and have him toss and drop into the racket drop (back scratch, whatever) and get the feeling of the position. he can toss, drop in, and hit, do two or three practice tosses for each attempted hit. As he gets it, start to do full service motion incorporating the new transition. I could get more technical, just trying to keep it simple, I'm sure you will get more feedback.
Wilson Ad Staff, 4 BLX Blade 98 18x20, Alu Rough/PolyFibreII. 2013 40+ 4.5 National Champs