Thanks for the additional information and answers to my questions.
Motrin is not a drug which I would give him for several reasons: It's not a great drug when diarrhea issues are present
He's already taking an nsaid so to give him another one runs the risk of gastric ulceration.
And, there's such a narrow margin of safety in dogs, that I rarely recommend that it be given for any reason.
Since he's taking Carprofen, this will limit what I typically suggest that owners give their dogs when they develop diarrhea...which is Pepto Bismol since it contains salicylates (the active ingredient in Aspirin). The risk with this drug is the same as with Motrin which is that there's the risk of double dosing him.
The following is what I advise my clients when their dogs have diarrhea and yet they're otherwise acting fine:
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.
2. I'd start him on a good quality, canine probiotic such as Forti Flora or Resources Protegrity GI. These products can be extremely useful for gastrointestinal issues (and to help strengthen the immune system as well). They may be available at local pet/grain stores or can be purchased online.
3. You could give Loperamide or Imodium (which is the drug I suspected you were thinking about) but if there's the slightest chance that he might have some Border Collie in him, I wouldn't risk it.
Collies, shepherds, related breeds, or other dogs who have a defective MDR-1 gene mutation may be overly sensitive to loperamide and it's use should be avoided. They tend to react in a more adverse way than other breeds so it would be safe to avoid its use if at all possible.
In fact, because of the potential for side effects in any breed, I tend to avoid recommending it (http://www.peteducation.com/article.cfm?c=26+1303&aid=1432)
But the dose would be 1/2 of a 2 mg tablet 2-3 times a day.
This is a tough situation since there really are no other over the counter drugs which would be effective for looser stools but it's quite possible that the bland diet and probiotic will be sufficient to firm them up.
I hope this helps and that you do see improvement soon in the consistency of his stools. Deb