Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear about this poor fellow's loose stools. He sounds like he has had a rough time of it.
Diarrhea can be due to stress, a change in diet, eating things he should not (garbage, dead animals), parasites, inflammatory bowel disease or even a food allergy. In an older dog it can also be related to metabolic organ disease (such kidney or liver disease) due to organ wastes irritating the gut or a mass in the intestinal tract.
If he seems to be bright and alert and has a good appetite I am less concerned. If not he should be checked by your regular veterinarian if he's not bouncing back quickly with the suggestions that I give you.
Well worth checking at least a couple fresh stool samples too as parasites are a common cause of loose stools in stray dogs and most parasites are microscopic. Parasite eggs/cysts are shed intermittently, so several stool checks may be necessary to find them. They may be picked up on the second sample or third test if the first is negative.
If his appetite is off and his stomach is gurgling he may have some stomach upset and reflux that can go along with loose stools.
To try and settle his stomach today you can give either:
1) Pepcid-ac (famotidine) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one 20mg tablet per 40 to 80 pounds of body weight every 24 hours.
These will reduce stomach acid and should help if this is related to simple nausea and gastrointestinal irritation. They can be given for several days if need be. If his appetite is fine you can skip this step.
I would not feed any food for 12 hours. This should help stop gut spasms and restore normal gut motility. Small amounts of water, low salt broth, or ice cubes given frequently are fine as he needs fluids after all that he has lost with diarrhea. You can give pedialyte to replace electrolytes too but Gatorade is much too high in sugar which can make his intestinal irritation worse.
Today even with the fast you can start Kao-pectate at 1ml per pound or 1 tablespoon per 15 pounds of body weight every 6 to 8 hours. This is quite safe and will coat his irritated gastrointestinal tract as well as absorbing bacterial toxins. You can use it for several days until his stools look normal. You can find kao-pectate at the drug store.
If he has a tense painful abdomen, continues to have diarrhea with no improvement after being on kao pectate for 24 to 48 hours, becomes very lethargic, begins vomiting or runs a fever greater than 103.5F then he really should be seen by a veterinarian as soon as possible. Make sure to take a fresh stool sample with you when you go.
After his food fast start a bland diet of 1/3 boiled, lean hamburger (or boiled, white, skinless chicken), all fats and juices drained off the meat, mixed with 2/3 boiled, plain white rice. Feed several small meals a day. You might wish to add 1 to 2 tablespoon(s) of canned pumpkin to each meal if you are seeing mucous in his stool as fiber helps soothe an irritated colon.
You can also add probiotics to his bland diet to replace the good bacteria lost with diarrhea. Good brand name products include Fortiflora or Benebac. They can be found online or at pet stores.
Once he feels better (no diarrhea for 48 hours) start mixing in what will be his regular dog food very slowly, less bland more regular with each day. It should take a week or so to convert him.
If he is vomiting and feeling poorly though it would be best that he see a veterinarian now as anything you give orally will just come back up worsening his dehydration.
Please let me know if you have any further questions.