What shall I do with my dog?

I usually don't write something that other users said before. But this time I feel that it's the only way to convince you to do something good for your dog's health: Take the dog to the vet right away. There's no time to lose, blood in the stools or black stools are an emergency.

Bring the dog to a veterinarian for a check up. He might have a Gastrointestinal bleeding if the blood is bright red the bleeding might be at the rectum or large intestine but if it dark it might be at the stomach. Better bring the dog to a vet before it is too late.

Bring your dog to a vet asap and also if possible bring sample of his stool. He may have parvo or other disease.

I must say that I am quite shocked that you have had to ask what you should do about your dog because it is bleeding from the back passage when he goes to the toilet. What would you do if you were bleeding from the back passage? I am sure that you would rush yourself to the doctors. So why is it that you didn't rush your poor dog to the vets. If you can't afford it then you should know that there are many places that you can take your dog too that is free. If you do not know this than I am afriad I don't think you are responsible enough to have a dog. Having an animal is a responsibility and from the day you got your dog you should have made yourself aware of what resources you may be in need of one day. I hope your dog will be ok and won't be found to have a life threatening desease, and I also hope that in future you will act faster and before having the time to visit your favourite internet site.