It sounds like more diagnostics are in order. Blood from the penis could be due to a urinary tract infection (bladder or kidneys), prostate infection or cancer, trauma to the penis, or infection of the penile sheath.
In other words an infection or inflammation anywhere along the plumbing leading to the penis can be the cause.
A culture and sensitivity performed on a flush of your dog's penis or his urine may be the best option. An X-Ray and/or Ultrasound of his penis, prostate, bladder and kidneys may also help.
While waiting for the culture and sensitivity results your vet could perform a therapeutic trial by switching to a stronger antibiotic such as Chloramphenicol.
Does anyone have any idea what it may be...PLZ!!