I had the same issue with one of my boys about ten years ago. We went through the same thing treating with antibiotics repeatedly. The vet would test and find bacteria and treat. They also advised that he be neutered, but I held off. Sometimes his semem would seem to be all blood. I read everything I could find and I did speak to Dr. Hutchinson about it at one of his seminars. While he could not diagnose my dog obviously without examining him, he did make me feel like things may not be as bleak as I thought. The issue went on for a couple of years, but it did seem to be subsiding (less blood in the semen over time), so I quit stressing over it and took a wait and see approach. The issue started when the dog was about three and completely subsided by the time he was about five. I am very happy to say that he had no fertility issues and was still producing nice litters until he was nearly twelve years old. He is now a very happy thirteen year old retiree.

I hope things work out for you and your boy. If you can get a consult with Dr. Hutchinson, I would highly recommend it.