It might have something to do with the "collection method"...if some slight abrasion happened during the collection?
If not, then it's a condition named "hematospermia". It can be a sign that there is a problem with the prostate gland, but is more likely to occur in an older dog....still, it can't be ruled out even in a young dog. It is possible to have an infection in the prostate. A course of antibiotics should clear it. Baytril (unless contra-indicated) is an effective antibiotic for prostate infection, but the course is longer than a usual 5-day course.
You should speak to the vet about this, and it may be the vet will test his semen for white blood cells, etc which show infection is present.
The sample you already have may have deteriorated by the time you make an appointment. so a fresher sample may be needed for analysis.
Discard the sample you already have as you should use a clear semen sample for impregnation (unless you collected it for fertility testing?)
Also look carefully at his scrotum for any signs of injury or damage or any unusual swelling.