Can you buy a used treadmill (they are often on craigslist for fairly cheap)? Put it at an incline (we do 6% for Nash) and walk him on it for 20 minutes a day. Swimming is great, but now its to cold for that. Trotting up hills helps a lot too. I take Nash to
Animal Rehabilitation Center of Rochester, mainly just because they are right near my house, but they have been helpful with Nash's issues (
http://www.germanshepherds.com/forum/stories/144502-its-his-hamstrings.html).
Getting a pulling harness and having him pull light weight will help too. Be careful with that though, keep the weight light. Walking up the stairs several times a day will help too, have him walk slow, one foot per step. Teach him to back up on command and have him walk backwards several feet a few times per session, a few sessions a day. You can make a couple of jumps for cheap from PVC, just keep them low and on grass, just above hock height is high enough and run him over those a few times a day.
It may also be helpful for him to have some core conditioning as well. Have him roll over, going to both sides, a few times a day can do a lot for his core muscles, which will help with over all condition. Teaching him to "sit up pretty" also works those muscles.
