Hi Kari and Leo and welcome to FDMB.
How long ago did Leo stop eating? Is he drinking water on his own? Are those BG's typical or much higher than his usual? Have you checked Leo for ketones?
I don't want to alarm you but with him not wanting to eat, a recent infection and those high BG readings, I'd be concerned about DKA (diabetic ketoacidosis) which is serious and potentially life threatening. You can get strips at the pharmacy to test for ketones but if you've never done this before, Leo might be wary of you trying to catch a urine sample. With that in mind, if you haven't tested for ketones and can't do so immediately, I'd contact the vet again and tell them you are very concerned about DKA. If that is the problem, the sooner you have him seen the better because not only is it life threatening but it can also be very expensive to treat. If your vet cannot see him until tomorrow, is there another vet clinic you could take him to?
In the meantime, you need to entice Leo to eat and I wouldn't be fussy about what you give him as long as he eats. Do you have a baby syringe that you can try to get some fluids into him? You can add water to his food to make a soupy mixture and syringe that into him if need be but I'd use a food flavour he's not usually fond of if possible so as not to create any aversion to his usual diet.