Now when we see a dog show diarrhea, we do have to consider a range of causes (just as with people). The most common reasons for a dog to show these signs are dietary indiscretion (eating something he shouldn’t have), viral infections (ie parvo, distemper, etc), inflammatory conditions (ie IBD), toxins, intestinal parasitism, cancer (hopefully less likely here) and a bacterial gastroenteritis. If he isn't a mischievous wee soul, then hopefully we can put worries like toxins and foreign bodies (which we' d want to address as soon as possible) lower on our list of concerns.
So, if we can put those concerns lower on our list, then you can try and settle his stomach at home. To start, I would also advise starting Sam on a light/easily digestible diet. Examples include cooked white rice with boiled chicken, boiled white fish, scrambled eggs (made with water and not milk), or cottage cheese. There are also veterinary prescription diets that can be used in cases of gastroenteritis, (ie Hill’s I/D or Royal Canin’s sensitivity). The easily digestible diet tends to be better tolerated and absorbed by the compromised gut. And if a infectious agent is wrecking havoc on the GI, then we want to be making his ability to gain nutrients as easy as possible for the gut. You want to offer small frequent meals, as this will also aid in decreasing diarrhea load. It he does settle on this diet, then we'd want to keep him on it for at least a week and then slowly weaned back to their normal diet over another week.
As well, since abnormal feces often leads to abnormally high levels of fluid loss, you need to keep a close eye on his hydration. If you are concerned that he is become dehydrated, then you do want to check his hydration. When checking a pet's hydration status, there are a few things we can test. One is whether the eyes appear sunken, if the gums are tacky instead of wet/moist, and whether he has a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a wee video on this HERE. If he is showing those dehydration signs at this point, that is our cue to have him to the vet since oral rehydration may not be enough to get him back on track.
Otherwise, we do want to keep him hydrated in the face of this diarrhea. To do this, we'd want to encourage Sam to keep drinking. Fresh water should always be available and low sodium chicken broth can be offered to tempt him. Furthermore, as long has he doesn't experience vomiting, you can can offer or even syringe feed him pedialyte (pr pediatric rehydration solution) or Lectaid (available OTC from your vets). The nice thing about these two is that they will provide hydration as well as the electrolytes that he will be losing in his diarrhea. A typical maintenance rate for hydration in an animal is 48 milliliters per kilogram of body weight per day. This value will give you the total he needs for the day (though doesn’t take into account diarrhea losses which you can add to this total) and is a good starting point to give you an idea of his daily requirement. If he vomits you given pedialyte, I would discontinue this as a therapy. (since we don’t want more vomiting because of our intervention).
Furthermore, there are some anti-diarrheals that can be used in dogs to slow things down for their gut. As I am sure you appreciate, these would not be a cure (since cures would depend on the culprit and might include antibiotics or anti-parasitics, etc.) but would slow the diarrhea to aid the body potentially absorb more water/nutrients then it would have if the diarrhea were unchecked. In regards ***** ***** options for your wee one, the one we most commonly use in dogs is Kaolin/Kaopectate (More Info/Dose) or PeptoBismol (More Info/Dose) available from your local pharmacy. Furthermore, Protexin Pro-Fiber (which is available OTC at vet practices; example) would be another option. All will slow diarrhea and the Pro-Fiber has the bonus of providing support to the delicate good bacteria of the GI.
Overall, when we a dog develops diarrhea, there are a range of issues we must consider. Still as long as he isn't passing profuse amounts of diarrhea and is otherwise well in himself, you can try him with a light diet and anti-diarrheal while monitoring his hydration. If you initiate these treatments and do not see improvement over the next 24-48 hours (especially since he is older) or he is appearing dehydrated, then I would advise following up with his vet at that stage so that they can address possible pathogenic causes of this diarrhea. If you need to do so, consider bringing a fresh fecal sample with you so that the vet can assess/test the feces and advise you on which differential is to blame for Sam's diarrhea. Depending on their findings, they can dispense the appropriate treatment (ie antibiotic if bacterial agents, anti-parasitics for protozoal or worms, etc) to get him back to his normal self.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,