Was he “trained” to a certain level? For example, verifiably schooling 1.0m for a few years? Or was he just in an eventing program. I also have had life actually get in the way, no fault of the horse, but with yours I wonder if he was NQR and they turned him out. Not saying the seller was nefarious or anything, just a thought. As someone else said, it is a different approach if the horse is being asked to do the same or less than they definitely were doing soundly before, vs being asked for different or more.
However, it probably doesn’t matter since “back to work” for a month doesn’t really tell you anything about his soundness immediately before coming to you. What it does say is that this horse may never have been fit enough for real work since he came out of the pasture - it sounds like the issues popped up rather quick? The NPA would be stopping him from building proper muscles, and then the stifle and back gets involved and it all goes haywire. BTDT, got the tee shirt!
I feel for you, OP. Unwinding these guys can be really draining, but SO worth it when you make some headway. They often have a lot of related aches and pains that need to be addressed all at once, which makes it hard to know what worked and what didn’t. Good luck! And keep us posted!