I searched and found this.
There are two things to think about when bleeding occurs from the penis.
The first is an injury to the penis or some other part of the urinary tract
leading to the hemorrhage. The second is a bleeding disorder that is
affecting the entire dog but showing up as bleeding from the penis. It is
important to note that occasional instances of small amounts of bleeding
from the penis (several drops but enough to be noticeable) are not highly
unusual in intact male dogs and often do not seem to cause any significant
problems. This is especially true when they are aware of a female in heat
but not able to breed with her. It sounds like you are seeing more bleeding
than this, though
http://www.vetinfo4dogs.com/dmale.html