Maybe say something like, "Under NO circumstances are you to shave him" --- I do most of mine's grooming myself, but about three x's a yr I take them in for a good clarifying/conditioning/"trim" (one gets the schnauzer cut though) I spell out everything each time I go in...no cutting of the snout/eye hairs... don't touch the topknot, pluck the ears, trim only the tips, only "blend" stray hairs, no major cutting, etc., etc. Even things that should be a given, I don't leave to chance. The schnauzer cut one is the only one I leave them to do as they feel best.
Sometimes groomers do a really good job a few times and having so many clients they can mix up who is who...best to just tell them - nicely of course because we all know how hair dressers and groomers get rid of annoying customers: one less than perfect cut usually does it.