If the blood is red then it can be from parasites or protazoa, dietary indiscretion, colitis, polyps or a possible obstruction of the bowel. If he is eating then you can rule out an obstruction. The boxer breed is prone to intestinal problems such as colitis, so you may have to change his diet to one that is more stomach sensitive. If you have changed the food at all then this can be a cause also.
I would take a stool sample in for testing to rule out parasites and protazoa, as well start a bland diet of boiled chicken and cooked white rice for a couple of days and see if there is improvement You also want to check the gums and make sure they are nice and pink, if they are pale or white he needs immediate assistance.
Red blood in stool causes
If the stool is black and tarry looking then it could mean the dog is bleeding from higher up in the system such as the stomach or upper intestines and the blood is being digested. If this is the case, then see your vet as soon as possible. Again ,check the gum color. http://www.petplace.com/dogs/melena-blood-in-stool-in-dogs/page1.aspx