This is an issue that should not be ignored. The types of things I consider in these cases include the following:
1) Penile trauma: Some injury either to the prepuce (the sheath) or the external aspect of the penis. This needs to be carefully evaluated.
2) Bladder or Urethral stones: Stones that form in the bladder can sometimes find their way part way down the urethra where they partially obstruct the outflow of urine through the urethra. These can also traumatize the urethra so that bleeding from the penis results. A urinalysis and x-rays need to be done to identify these.
3) A urinary tract infection: UTI's can result in bleeding occasionally. A UA needs to be done to identify this.
4) An infection of the prostate gland: In the male dog, prostatic infection can occur at the same time as a UTI.
5) Prostatic, urethral, or bladder tumors: These can cause bleeding as well. X-rays or ultrasound may be necessary to diagnose these.
6) Problems with clotting mechanisms: Bleeding abnormalities, rat poisoning, DIC and other things can result in failure of the clotting mechanisms that can result in bleeding from the penis. These can be very serious problems. They are diagnosed by doing mucosal bleeding times, PT, PTT and platelet counts.
So I would take your dog in and have him evaluated for these problems with the tests mentioned above. Do not delay.
Hope these thoughts are helpful.