I had my mare for about a year before she got her first bloody nose, and with her it was a recurring issue. She never actually raced, but the vet said that some horses just happen to be bleeders. It is more common in race horses, but he said that in her case it was coming from her lungs. As she worked, and started breathing harder, occasionally she would start bleeding from the lungs, which naturally came out her nose as she breathed. It was nothing major, and he said that unless it started coming from both nostrils (it only came out of one at a time), or was very profuse, to not worry about it. Cool her down a bit, give her a break, then continue riding. I personally would have your vet out to look at him though, just to be sure there is nothing to worry about. They can't make a guaranteed diagnosis over the phone, so everything is just speculation until the vet actually takes a look at him.