Dogs can take pepto bismol. You give 0.5-1ml/pound. A tsp equals 5ml. If the dog is acting weak and not himself you probably should bring him to the vet. Diarrhea is not as dehydrating if the dog is drinking okay but it can lower their electrolytes. So make sure the dog is getting enough water. You also can give the dog pedialyte, the same thing you give to babies when they are sick. I hope this helps you.
More than three days and you need to see a vet, it probably is only going to get worse without treatment when that happens. If there are any other symptoms such as blood, black, yellowish, greenish, slimy, gel whatever you need to go sooner than 3 days. Since it's been more than 3 days you need to see your vet. Some possibilities are parasites, parvo, SIBO, kidney problems, other viral or bacterial infection, foreign blockage and a whole other host of things. Treating at home by this point could cause your dogs problem to be FAR more serious especially if it has something to do with an organ and you give the wrong thing or poison them accidentally.