
Is it definitely in the semen?
I agree that a vet check is DEFINITELY in order, but something possibly vaguely relevant happened to my entire male some time back.
He was oversexed for one reason or another, including our bitch being in season, and on a bit of a blanket hump session one evening I noticed a spot of blood on the floor under him. I dabbed him with tissue and there was a bit more blood.
Long story short, my vet said that occasionally when they get a bit over excited and on a hormone surge they can burst a small blood vessel in the penis which bleeds.
This was just a few drops. No problems before or since.
I'm not sure about blood in the actual semen. Blood coming from the penis of an entire dog can be prostate problems. Get him checked out then see where you can go from there.