Thank you for the additional information.
There are many possible cause for diarrhea and often these can also cause a loss of appetite as well' the most common causes would be:
*Intestinal parasites. Not just worms but also microscopic protozoa such as coccidia and giardia Click here
* Dietary indiscretion - Eating something they should not
* Diet changes
* Bacterial infection.
Considering his age if the problem lasts more then 24 hours or he declines at all he should be seen by a vet.
Some things you can try at home for today are
1. Pepto Bismol. This will help with any stomach upset and the diarrhea. This is dosed at 1 teaspoon per 20lbs every 6-8 hours.
2. If you do not have Pepto, the Pepcid AC will help to calm the stomach but it will not help with the diarrhea. A small dog can have 1/4-1/2 of a 10mg tablet every 12 hours
3. Offer a bland diet of either boiled chicken or hamburger and white rice . This is easily digested and will also help reduce the diarrhea.
If he refuses to eat by tomorrow morning or declines in any way I would suggest having him seen by the vet. The vet may want to do blood work, fecal testing or x rays depending on what they find on physical exam.
I would try offering the bland diet and either the Pepcid or Pepto and hopefully he is just having some digestive upset that will resolve. Please reply back if you have additional questions or concerns and I will be happy to continue.