Dogs may have a little bleeding or discharge from the rectum for a couple reasons: damage done to the rectum while defecating, certain types of cancer, impacted/infected anal glands, parasite infection and polyps are just a few.
If there is only a small amount of drainage around the area, I would just keep an eye on it for now. Sometimes dogs will try to pass a large amount of fecal material and can cause small tears in their rectum. These generally will heal on their own without any sort of medical intervention. Feeding the dog some canned pumpkin (make sure that it's only real canned pumpkin, not the pie filling) can help make the stool a little easier to pass, thereby helping to not tear things any further.
Parasites such as worms and protozoan infections such as coccidia and giardia can produce blood in the stool and blood from the rectum after bowel movements. You can read about these here: http://www.thepetcenter.com/exa/worms.html http://www.peteducation.com/article.cfm?cls=2&cat=1622&articleid=727
A problem with the anal glands can also cause a bit of bleeding. You can find more information on those here: http://www.dr-dan.com/analsac.htm .
You can read about rectal polyps here: http://www.petcancer.co.uk/cgi-bin/articles/020.htm
Whatever the cause, rectal discharge is never normal and I think it's a VERY good idea to give your vet a call when they open and have them take a peek at him, just to be sure that this isn't something more serious than just a little tear from defecating.
I hope this helps!
Just going to the vet doesn't always mean they test for parasites...but they could have.
It's not unusual for a dog to have bleeding if they've had diarrhea. The force of the defecation can cause tears in the rectum that are quite severe and can deposit a lot of blood on the feces.