Get this poor horse checked for KS, if nothing there then start at poll and work all the way down his spine to the tail head.

Mine I posted above about also palpated fine, passed a lameness exam no problem. Refused to lope right, got worse with time. No pain killers made her better either, they still don't. My friend that does her PEMF just happened to have a thermal attachment for her phone for imaging. White hot spot on her spine just behind withers, took it with to Langer at WEC and we xrayed that spot. There was KS and old fractures there. That was October, still working on her now. Injected 4 of the spinal processes in May and just did her SI Friday from what we assume in compensation soreness. If your horse does have KS then be prepared for a long never ending road of maintenance, even after surgery. If I can get mine on the pattern I will most likely only run her long enough to determine if she's worth breeding or not & be done 

Have you looked into PSSM - more likely than not, PSSM2 being it's started later, vs when horse was first broke out, ridden, etc.  Since the vets are not finding anything "physically" limiting his movement, I would sure look into that.  University of MN is doing a study right now, wanting horses with symptoms and a control horse w/no symptoms. If you ask your vet about PSSM2, chances are they are going to say only way to diagnose is muscle biopsy but unless horse has been tying up, it probably won't show anything.  A company called Equiseq DOES do the dna via hair follicles - some dispute their testing since they are not "Peer Reviewed" yet, but that is in the process and will be proven once U of MN's study is done.  OR you could start "treating" him like a PSSM2 horse and see if any of the chances helped at all.