If you have any doubts, please call a vet or take him to an ER vet asap.
Sarah, I would ask this question on Health if you haven't already. I have not dealt with ketones, but I know the response depends on several things: How "positive" were they (slight, moderate, etc.)?, What is your cat's experience with them? Has he had them, been hospitalized for them, how well did he recover, etc. How experienced are you in dealing with them?
IV or subQ Fluids are given, fast acting insulin is given to get the BG down, but also it is important to get the electrolytes back in balance as fast as possible. and it is important to get nutrition into him. If he won't eat well, then a glucose drip (and more insulin) is given or a feeding tube in put in.
There are people here that have managed their cat at home because they have been through it before and the ketones are slight or moderate, not high. This is ONLY to be attempted if you know what you are doing.
Ketones can turn into full blown DKA in hours.