Answer

This may be due to intestinal upset from a change in food, intestinal parasites such as worms, coccidia, or giardia, or he could have a viral infection starting too if he did not complete his puppy shots.

This could also be due to a swallowed non food item, ingestion of something toxic including plants, mushrooms, chemicals etc. A swallowed non food item is pretty common problem in this age pup due to teething and chewing on things too soothe any jaw aches from adult teeth coming in and settling into the jaw.

So seeing the vet to find out what is going on should be a priority.

An over the counter med you might try would be Pepcid or Tagamet or Zantac.

Be sure to tell the vet when you see them if you use any of these.

You can read about dose amounts (and cautions) scrolling down on the pages here

http://www.petplace.com/drug-library/cimetidine-hcl-tagamet/page1.aspx

http://www.petplace.com/drug-library/famotidine-pepcid/page1.aspx

http://www.petplace.com/drug-library/ranitidine-hcl-zantac/page1.aspx

If that helps then you can see if some bland food and plain pedialyte added to the water bowl will work to resolve the upset.

I would suggest trying boiled chicken meat or hamburger with the fat drained off, mixed with really well cooked rice in a ratio of 1/3 meat to 2/3 rice to see if that works. It's not a balanced diet but OK to use for a few meals while working on controlling digestive upset.

If your home care doesn't work over the next day or if he seems worse when your vet opens I'd suggest you see them.

Hope this helps you!