The pictures aren't very clear but it appears to just be a minor injury to me. He could have scraped it on the fence or a tree or he could have rolled on a rock. Horses can injure themselves on invisible dangers. I wouldn't worry about it. It will heal up just fine.
Always remember that feeling of looking at a big, open country over the ears of a good horse, seeing a new trail unwind ahead of you, and that ever-spectacular view from the top of the ridge!!! Follow my training blog: http://robertsontraining.blogspot.com/