If he won't drink liquids, can you do an end run around it and feed him liquidy things like soup, jello, and lots of fresh fruit (particularly melon and citrus) and vegetables? Or alternately offer things like pretzels and unbuttered popcorn at snacktime to make him thirsty? Have you tried fruity water drinks? Most are pretty sweet and some are fizzy like Coke.
It's a challenge to keep Maya in enough fluids as she sweats like you would not believe (there are always sweat rings around her head on her sheets), but we just got into the habit of carrying a sippy of water around all day. Of course now, it gets me trouble if I try to run a quick errand and don't bring a cup.