He might be expecting pain when someone is onboard, so you need to regain his confidence that a rider wont cause him pain. You are perfectly correct in going back to lunging and long reining, I would start without any tack (except cavesson/headcollar) and go from there.
Remember, a the back-man isn't a one-stop-shop, he'll probably need the physio out periodically in the future. Get in contact with the person who has recently done his back, and talk about perhaps having future visits, say every half year or summet. Idk much about physio, so ask the professonal