Thanks so much for the additional information.
As you probably already know, there could be any number of reasons why a dog this age might develop diarrhea ranging from viral/bacterial infections to dietary indiscretion to garbage gut to internal parasites to Inflammatory Bowel Disease...and this is just a partial list.
Most over the counter wormers are fairly effective at killing roundworms and possibly hook worms but they often don't target whip worms or giardia.
So, even though you wormed him, internal parasites may still be the problem, if that makes sense. I often want to rule out this particular problem when a dog has a normal stool followed by a looser one. This pattern is also frequently seen with Inflammatory Bowel Disease which is often associated with the diet (usually one which has been fed for a while).
But his pattern could be consistent with other conditions as well.
However, having said that, most dogs with loose stool issues will respond to symptomatic treatment although some of them will require stronger antidiarrheal drugs or additional worming from the vet. The following would be my suggestions which may get him back on track in a short period of time:
1. Feed a bland diet of 1 part boiled chicken (or hamburg) to 4 parts 20-minute white rice. Feed only a small amount of one time, 3-4 times through the day. I would not feed any regular dog food or cereal.
2. If he is not currently taking any aspirin or nsaid products and is not vomiting, then you can give him Regular Strength Pepto Bismol if available to you.
The dose would be 1/2 ml/lb 2 times a day. For example, if he weighs 50 lbs, then you would give him 1.5 tbs at each dose (15 mls=1 tbs). It may change the stool color to a gray-black or greenish color though so this shouldn't concern you if it happens.
Pepto bismol should not be given to vomiting dogs since it contains salicylates (the active ingredient in Aspirin) which can irritate the stomach.
3.Consider probiotics such as Forti-flora.These may be available at local pet/grain stores.
If his stools remain loose after 2-3 more days, then he might need to be seen. I'd take a stool sample with you so it can be evaluated for parasites that I mentioned.
I hope this helps and that his stools firm up quickly. Deb