Answer

It sounds like your dog has hemorrhagic gastroenteritis.

Hemorrhagic gastroenteritis is an acute onset of bloody diarrhea in formerly healthy dogs.

The cause is not known but they think it might be caused by diet or an abnormal response to bacteria. King Charles Spaniels, Pekingese, Poodles, Schnauzers, and a few other breeds are more frequently affected and the age of the dog is not a factor.

Factors which might cause the bloody diarrhea include food allergens, bacteria, or intestinal parasites.

It's important to make sure your dog drinks sufficient fluids and eats so that he won't become dehydrated. Offer him Pedialyte. We've found that in the summer our dogs like a few ice cubes in their water bowl.

You can try feeding him a new protein food to determine if the cause is a food sensitivity e.g. try feeding him cottage cheese, lamb, and rice for 2 weeks to see if the bloody diarrhea subsides.

He may require IV fluid therapy with potassium and antibiotics.

There are other possible causes of GI bleeding that should be ruled-out so it's good that you will be taking him to the vet.