I am a vet tech. You should take him to the vet because he could be trying to pass stones or have an infection.

 The bleeding from penis in dogs can be due to injury on penis, infection in urinary tract, infection in prostate and stones in the bladder. If the bleeding is mild and occasional especially when he see female and can not have sex, is normal. But bleeding other than this condition needs to be investigated by Vit.

 It can be because of a bladder or prostate infection. It can be because of bladder stones & X-ray should be able to show that. If bleeding isn't stopping then kidney or liver function checkup along with platelets & blood cells counts is needed. Bladder or prostate cancer can also be the reason, If there is too much non-stop bleeding.

 Bleeding from the penis is never normal.  It can be related to a urinary tract infection, prostate infection (much more common in intact males), kidney infection, bladder stones, kidney stones, tumors (intact dogs can get transmissible venereal tumors), trauma, or from spontaneous bleeding often from a toxin exposure like certain types of rodenticides.
Have your dog examined by a veterinarian.  Tests may include urinalysis, radiographs, and/or ultrasound.