Thanks so much for the additional information.
Charlie sounds like a lucky cat to have found your sister to care for him. I can understand why she might be so concerned about him with the symptoms that you describe.
It sounds like he could have multiple issues going on, given his age.
1. Diabetes is certainly one of the first diseases that came to mind since carrying extra weight is definitely a risk factor for developing this condition.
Even though he was tested last year and was negative, it's possible that if he were retested now, he would be positive.
But there are other conditions which can cause a cat to drink more water and urinate excessively (and which might have diarrhea as a secondary problem) such as kidney or liver or pancreas issues. Cats are masters at hiding disease until it catches up with them; even if his blood work was normal last year for these conditions (if it was done), things could have changed between then and now.
2. His difficulty walking could be secondary to his weight but it could also be secondary to arthritis. Treatment options for arthritis in cats are more limited than for dogs but supplements which your sister might consider would include:
a)Cosequin for Cats which is a joint supplement available online or in some pet/grain stores.
b) Fish oil supplements such as Welactin which is liquid that can be drizzled in the food. Also available on the internet. .
c) NuCat Senior which is a tasty gel and a source of antioxidants to reduce damage to joints
I don't include drugs since a vet would have to prescribe them; I don't know if this is something your vet would do without a recheck.
3. His loose stool issues could be secondary to systemic disease, as I mentioned above, but it could also be secondary to late onset Inflammatory Bowel Disease or cancer, unfortunately. And, of course, the different foods that she's trying to get him to eat could contribute to the problem as well.
I wouldn't worry too much about the blood, though. While concerning, it doesn't have to be alarming. In most cases, it simply reflects an irritated colon.
I also typically suggest boiled chicken for these cats but she could also offer human baby food (just avoid those with onion and garlic in them).
Probiotics such as Forti Flora can also help these cats with looser stool issues. It's a powder that can be sprinkled on the food every day.
I agree with you that something a little more serious may be going on with him; I always become concerned when older cats start to behave as you describe.
But, hopefully, the above suggestions will give your sister other options to consider for him.
I'm not sure if it's his time just yet if he's still engaging with her and still has a good quality of life but if he stops eating and becomes lethargic, then it may be, unfortunately.
I hope this helps although this is obviously a very sad situation for everyone. Deb