Just my thoughts, if your kitty's glucose is that high, I would say he's most likely diabetic. If you want peace of mind, you can purchase both a glucose meter and blood ketone meter to test him yourself for accuracy. This is my second go-around with feline diabetes, and I can tell you both times it was scary. The first time more so, as I had to teach myself the ins and outs of testing. But kitties are pretty resilient, and you'll learn some tricks that work for you. Nico was hardly phased by testing, and would let me do it anywhere anytime lol. Sadie, well she's more of a pistol. I have to put a churu on a plate to distract her, but now I'm quick and have everything laid out so it's done in like 15 seconds. Same with the ketone meter, even though it requires a little more blood for the test. It might sound daunting, but I think either way you view the situation kitty will most likely need some medicine to help regulate the glucose, and both meters come in handy for regular insulin and Senvelgo.
On the plus side, this forum has lots of posts reviewing different meters, how to test, and various tricks of the trade. I found it to be a life saver sometimes, as Nico's journey was full of ups and downs. I hope you get some answers soon to ease your mind, or confirm your suspicions (I hope it's the former!)