Yes, he's been out of work since the fall, and was just ridden a few times a week on the trails then. I plan to trail ride him, expose him to all sorts of stuff including endurance ride camps (I'd enter the 10-mile fun rides), and if he and I are both fit enough, possibly try a 25-mile ride by the end of summer.
If he is either over at the knee or tied in at the knee (are those opposites of each other?), how would that affect his soundness and useability for trail riding?
Oh, and I don't think he's all that downhill. I used my photo software to "straighten" both side-on photos. For the top one, I used the two further-away feet (his left feet), because of his right rear being wonky. In the lower photo, I did use the two closer feet, so I think it's more representative, and voila--he looks much more even.