Could he have eaten something bad?
Do you have any Fortiflora you can give him? That's a probiotic for cats. You can only get it from vets or online. It works well for helping reestablish the gut bacteria that he should have. It won't cure the diarrhea if there is an infection, but it might improve things some.
What happened with your vet discussion about his blood sugar, the dose and possibly switching insulins? He is extremely high.
I'd encourage you to get Ketostix and test him twice a day for ketones. Being sick + not having enough insulin are 2 of the 3 components that can cause ketones in a cat. The last component is not enough food, so keep him eating.
How is his behavior?