Thank you for your response. I will do my best to help.
It seems that you are picking up on some non-specific clues that your kitty is ill. Lethargy and decreased appetite in an older cat may be attributed to several problems. A scruffy coat likely means he is not grooming.
I would recommend separating the ill cat from his housemate to see if you can determine how much he is eating and drinking and ensure he is using the littler box. If he is truly not eating AT ALL, this is a BIG PROBLEM in an overweight cat. Overweight cats that do not get sufficient calories are prone to a disease called fatty liver. It can take as little as 2-3 days for this condition to develop. Fatty liver is not the primary disease, but can be a complicating factor to a myriad of other problems. If he is not eating this afternoon/evening coax him with boiled chicken, tuna, baby food (make sure the baby food doesn't have onions in it). If he still doesn't eat tonight, he needs to see a veterinarian ASAP.
Also keep a close eye on his litter box. Frequent trips to the litter box without urination (no wetness/clumps) are very concerning. This could be a urinary blockage which is an EMERGENCY!
Common things happen commonly in cats. The biggest reasons that I see older cats to stop eating include kidney
problems, hormone imbalances (diabetes
, thyroid problems), and pancreas problems. Many of these can be diagnosed with a thorough physical examination and bloodwork.
I would recommend calling your veterinarian to get an estimate for a physical examination and a bloodwork panel. There are credit lines (such as CareCredit) that can help. Getting veterinary help sooner rather than later can help to save money in the long run.
Please let me know if I can provide any further assistance.