Sorry, I had put "he now has lifts" and didn't detail enough I guess. Yes, the farrier has seen him as well as the vet and he now has a lift on the short leg. We've been working on strengthening exercises for the right side for a few weeks and he's much improved in that before he was very difficult to get into the right lead at all, and when he did he'd generally cross canter and slip back. Now he's consistently taking the right lead, so that is great improvement.
It may be rider error on the depart, perhaps leg placement on that side... however, he starts to drift at the half halt that precedes the actual cue, so there's something he's expecting. I may not be able to figure out what I'm doing until I get in for another lesson so somebody can actually observe, hopefully next week.
He also continues to do circles way off the rail long after the cue... and as well, will try to drift at the trot, though I can correct that more easily than I can at the canter. I did find that doing the arena as a square, with corners, rather than an oval did push him over MUCH better and he seemed to get the idea.
Usual cue, outside leg just behind the girth, but he is pretty well voice command trained so it doesn't take much pressure.