I'm sorry to hear about your pup's dark, loose strong smelling stool.
It's tough to tell from the picture but his stool looks dark brown in color, although it may also have a reddish tinge to it. It certainly isn't normal.
Dark/black liquid or tarry stools could indicate digested blood in the stool.
Diarrhea can be caused by a primary viral, bacterial or protozoan infection, or eating something that irritates the gastrointestinal tract, inflammatory bowel disease or a severe case of parasites, an obstruction or twisting of the bowel leading to intestinal wall damage, internal organ failure or cancer.
The danger with blood in the stool is that means the intestine is raw and a raw intestine more easily allows bacteria into the abdominal cavity itself and can lead to peritonitis. The darker the color of the stool indicates the blood in the stool would be from farther forward in the intestinal tract.
Can you check his temperature for me? Normal dog temp is 102F. A fever is >103.5F.
Can you also check his mucous membrane (gum) color? They should be bubblegum pink and if you press on them they should blanche (whiten) then return to pink in less than 2 seconds. They should also be moist not sticky. If they are sticky that means he is dehydrated.
If his color is fine and he doesn't have a fever or a subnormal temperature (less than 99F) then he is more stable than a dog with a high fever or a low temperature, but I still would recommend an emergency examination tonight if he starts to have very black stools, or pure blood stools and is very lethargic and/or vomiting.
If he has a fever, his mucous membranes aren't pink, and when you press on his abdomen it seems very tense or painful I would be more even concerned. These could be signs of a blockage, pancreatitis (inflammation of her pancreas causing increased enzyme production, usually related to a high fat meal) or possibly a viral infection. These are emergency situations and he needs to be seen immediately.
If he cannot be examined tonight do not try to feed him any more. Food fast him for at least 12 hours. This may stop gut spasms and help restore normal motility. Right now hydration is important because of the diarrhea so make sure he's drinking well. It's a good idea to offer pedialyte too in a few hours to replace electrolytes. If he won't drink on his own you will need to dribble it in with a syringe (2 to 3 teaspoons at a time). If this goes well (no vomiting) then in 12 hours you can try a bland diet. If he vomits even without food then he needs to see a veterinarian for diagnostics (bloodwork and radiographs and/or an ultrasound of his abdomen)
To make a bland diet boil hamburger (or chicken) chunks, drain off all fat, and mix 50:50 with plain boiled white rice. Feed 2 tablespoons initially. If he handles that well then feed him 1/4 cup of the mixture a couple hours later. If he handles that fine then feed him 4 meals of this mixture daily until he is feeling herself (usually in 2 to 3 days) and the diarrhea is resolved. Once he is eating well for a few days with no diarrhea start mixing in his regular food and slowly (over 7 to 10 days) convert him back to his regular diet by adding more of his regular food and less of the bland diet each day. If he starts vomiting, runs a fever, has a painful abdomen or stops eating he needs a veterinary examination.
You can also give him Kao-pectate to coat his intestinal tract and absorb bacterial toxins at 1/2ml per pound every 8-12 hours or 1/2 tablespoon per 15 pounds of body weight every 8-12 hours. This should help stop the diarrhea.
If he is vomiting, has increasing amounts of frank blood or dark tarry stools (suggesting digested blood), has a painful/tense abdomen or a fever (anything greater than 103.5F) or below normal temperature then he needs to see a veterinarian as soon as possible as this suggests more serious disease that can't be treated at home.
Either way I would suggest submitting a fresh stool sample to his veterinarian to check for gastrointestinal parasites.
Please reply back to this post with any further questions.