Glad to hear his trot is coming along! I don't think that shoes will help him in the back, but I guess you could always talk about corrective trimming or shoeing with your farrier and have his feet evaluated. Sometimes it is very tiny things that set something off to 'weird' and when we figure out what those things are we almost want to smack our heads and go 'That's IT? Why didn't I think of that?!'
You'll figure it out. Just keep working him slowly and methodically and hopefully he'll start to 'feel' when he's messing up his legs. Remember though that he is older, so theres always the possibly that he's just doing it out of habit.