Sounds like he has an abscess, and the tooth may have been really bad for the vet to have taken it out. He probably doesnt feel like eating if he has an abcess - you are right to ensure he has some water, so keep up with the sryringe. You could try him with some pilchards or sardines as they are very tempting for cats, you could even try and smash it up and try putting some in a syringe too, but if he doesnt pick up tomorrow I would take him back to the vet as he may need a course of antibiotics.