Pets

Cats can get diabetes. They can have heart failure. It's possible that he's whining about his loneliness and not about food. You could end up making him sick or killing him, and the food will not relieve his sadness.

I know that it hurts you to see him hurting. Ask a vet for advice on how to help the cat deal with his loss. Try giving him lots of extra attention. Maybe visits from a cat or even a dog that belongs to a friend or neighbor would help. Or new toys or cat videos. Something to help him direct his attention away from his sadness.

Good luck with helping him get over grieving. Give him some hugs for me.