A few months ago my vet ultrasounded the sacroiliac and lumbrosacral joints of a filly here. The ultrasound showed extensive damage to the ligaments of her SI and LS joints. Since this is a relatively new application of ultrasound technology there isn't enough data to use the information we had to make any kind of prognosis. I was not optimistic, however, I'm happy to report she is developing muscle over the injured area and her movement is improving every day. She actually did a floaty trot across the pasture the other day.
My vet told me that back lamenesses were not even taught in vet schools until after the California Racing mortality studies. So there are those vets out there who have never developed an eye for hind end lamenesses and don't really acknowledge them. Both my vet and my chiro told me that the filly with ligament damage could have easily passed a ppe when she was at her worst.
Anyway, I guess my point is, if your horse is happy going x-country even though he's obviously not quite right behind, imagine what he might be if you can manage his care so that he heals.
Good luck with him, he sounds like a keeper.