Oh Thelma, I'm so sorry. I know how stressful it is (for both you and Sinny) when our kitties are sick.
Heads up that he may get super lethargic on the Cerenia. That happened to Marshmallow. ECID, so he may be fine. If he doesn't do well on Cerenia, Marsh does really well on Ondansetron (name brand is Zofran but it's cheaper if you get the generic). Dose is 1/4 tablet up to twice a day.
Hopefully the Cerenia will help. He may have developed some IBD. There seems to be some crossover with diabetic kitties and stomach trouble. He could have developed a food sensitivity. It can happen even when a kitty has been eating the same kind of protein for years, kind of like how some humans develop a sensitivity to dairy or wheat even though they used to eat those things without any problems.
Let us know what the lab results are. If there's no obvious culprit when you get the results, you could experiment with different proteins to see if he does better on a different kind. Best of luck over the weekend until he goes back in for labs.