Okay, thank you for your patience, and for the details--that helps a lot.
Bloody discharge from the penis, in an older intact male dog, is most commonly caused by a problem with the prostate gland. In older intact males, the prostate gland is commonly enlarged (benign prostatic hypertrophy). In some cases, the prostate can become diseased, such as with a prostate infection (prostatitis or prostate abscess) or much more rarely, prostate cancer. A typical sign is bloody discharge from the penis. Other causes of this type of discharge can be a urinary tract infection or bladder stones, or disease anywhere along the urinary tract (kidneys, bladder, urethra).
Here is a great article that talks about prostate issues in particular:
Dogs with prostate infection may be feverish, and may have an enlarged, painful prostate with a rectal exam. Sometimes the only signs can be discharge, but this can progress to the dog feeling very ill. I have also seen diarrhea go along with this. So I would be very suspicious of this first in your dog, given his age and symptoms. The treatment for prostate infection is antibiotics, and typically these dogs improve rapidly.
As for how much of an emergency it is, this would depend on if he seems ill. It sounds like right now he is doing fine, outside of the discharge and a bit of diarrhea. You certainly can continue to observe him this weekend and be sure he is not acting more ill. If he is eating, drinking, normal energy level, and the dishcharge is not worsening, you can probably safely wait. However, if he is acting sick, lethargic, painful, not wanting to eat or drink, or the discharge or diarrhea greatly worsens, I would get him in this weekend.
I hope this helps, and please let me know if you have any further questions. I hope he will be all better soon!