Hello, I'm sorry to hear that your pup isn't feeling well.
Diarrhea means that the gastrointestinal system is irritated. If his colon is affected you may see mucous or bright red blood in his stool, the darker the blood the more digested it is and the farther up the gastrointestinal tract it is from.
This can be from a bacterial or viral infection, eating something he should not have (toys, bones, bugs etc), gastrointestinal parasites, a dietary allergy or sensitivity, and a quick change in diet. Because he is older inflammatory bowel disease or cancer are other possibilities.
If he still feels well though we can try some things at home.
I would not feed him any food for 12 to 24 hours. This should help stop gut spasms and restore normal gut motility. Small amounts of water or ice cubes given frequently are fine as he needs fluids after all that he has lost with diarrhea.
Today even with the fast you can start Kao-pectate at 1ml per pound or 1 tablespoon per 15 pounds every 6 to 8 hours. This is quite safe and will coat his irritated gi tract as well as absorbing bacterial toxins. You can use it for several days until his diarrhea stops. You can find kao-pectate at the drug store.
If he has a tense painful abdomen, continues to have diarrhea with kao pectate, becomes very lethargic, begins vomiting or runs a fever greater than 103.5F then he really must be seen by a veterinarian as soon as possible.
After 12 to 24 hours start a bland diet of 1/2 boiled hamburger (or white skinless chicken), all fats and juices drained off mixed with 1/2 boiled, plain white rice. Feed several small meals a day.
Once he feels better (no diarrhea for 48 hours) start mixing in regular dog food very slowly. Less bland more regular with each day. It should take a week or so to convert himr back.
You should submit a fresh stool to his veterinarian for a parasite check. Parasites are a very common cause of diarrhea.
Let me know if you have any more questions.