It's hard to say what else you should be doing as you're aware of the things that need to be done, ie blood and urine tests. The results of these will give you a better picture. If he's drinking so much he'll be using the litter box a lot too so if you can't get ketone strips, at least get a pee sample and take that to the vet and ask them to test for ketones as a priority. And do try to get blood for a bg test - from everything you say it looks likely that he is pretty high and he really should be getting insulin. He's probably not hungry because he feels rotten but if you can tempt him with a favourite morsel - anything - and get him to eat, you can shoot.
How long has it been since you noticed these symptoms? Cats are very good at hiding illness and he may have been diabetic for a while before the dx so the sooner you can establish a testing and shooting routine, the better he'll start to feel. It may feel like a mountain to climb right now but once you start it becomes much easier.
