Sooo sorry to hear about your kitty's passing. Diabetes is a serious disease in both cats and humans. I hope your dad gives this some thought.
I give my cat an injection of insulin twice a day... Same as humans use, if their diabetes is severe. It's simple to do. "Nacho" is so used to to the...
I give slippery elm daily to my almost- long-haired cat. It is fromulated by a vet. I get it in tablet form at the pet shop, and, though it is dry and unappetizing in appearance, he thinks it a real treat! No more hairballs, but no worries if I forget to brush him often enough!
It very well be diabetes. I have a diabetic cat, myself, and the symptoms fit. Is he also peeing a lot? If so, That's because the kidneys are not doing their job, the water he drinks just runs right through, leaving toxins in the cats body instead of eliminating them!
But most important...