I'm so sorry you did not receive an earlier answer. Different experts are online at various times and I recently logged on and was notified of your question. Your patience is greatly appreciated!
My heartfelt condolences on the recent loss of your other cat.
Cats grieve when they lose a companion, just as humans do, so Jasper's loss of appetite might be related to the stress he feels due to this loss. You can get a Feliway plug in diffuser to help diminish his stress/anxiety:
The fact that he's diabetic, not eating and is lethargic could be related to ketoacidosis, a side effect of metabolic imbalance that is possible with diabetes. See if his breath smells like nail polish remover; this is a sign of ketoacidosis and is an emergency. He also may be hypoglycemic (his blood sugar is too low) because his diabetic condition has changed and his insulin dosage needs to be adjusted or the brand of insulin needs to be changed.
His blood sugar should be tested (I don't know if you test at home), so it would be advisable to have him seen in person by a vet or ER vet center, now, to find out the cause of his symptoms and of course, do NOT give him any insulin if he has not eaten.
I hope you're able to have him seen as soon as possible, you will receive a diagnosis and Jasper will be feeling back to himself, very soon!
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