If he is not eating or drinking he may be dehydrated that is an important situation to seek veterinary care for.
It may be he swallowed a non food item that is obstructing his intestinal tract or he has a twist or telescoping of his intestines causing this.
An over the counter med you might try to see if it helps if this is just upset due to eating bad food would be Pepcid or Tagamet or Zantac.
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
You can add plain Pedialyte to the water bowl to help with hydration. Or use a spoon,syringe, or sports bottle to give him some right in his mouth.
You can also see if bland food will help your pet feel better.
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.
BUT!
If your dog might have gotten into anything toxic, is getting dehydrated (see if a pinch of skin over the shoulder blade stays stuck in a tent form or flows back down normally when released), has gums or tongue that look white, blue or yellow tinted where they are normally pink, or is becoming lethargic and non responsive to you then I'd pack the dog up to see the emergency vet if one is available in your area.
I would plan to see a vet tomorrow with him at the very least to get him fluids, anti nausea meds a check of the stool for intestinal infection with coccidia or giardia and maybe an x-ray to see if there is a swallowed foreign object causing this stomach upset.
He's young and he is a small dog and they don't have a lot of reserves if he is getting dehydrated that can get serious very quickly.
*If* you think this is due to an insect sting or bite then you can see if the dog can hold down some plain Benadryl at a dose of 1mg per pound of dog. The tablets usually are 25mg and so a 10-12 pound dog could take 1/2 a tablet.
It could make the dog lethargic though if it isn't needed.
Hope this helps you!
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