Good Morning and welcome. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
Because of Scratches age, I wonder if he is suffering from Inflammatory Bowel Disease. If this is the case, there are a few things that you can do that may help his issues.
Purchase Probiotics at the pet store and give him a dose in his food every feeding. This will help digest the food that he is eating and may resolve this issue. I would also recommend that you change his food slowly over a 2 week period of time to one for a sensitive stomach such as Royal canin for cats with sensitive stomachs. this may help Scratch past this.
If this does not help, then your vet will have to prescribe medications for him.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, Dr. Loretta