Definitely check saddle fit and have a chiropractor out. My mare had the same problems. We had to go with a thinline with shims to correct saddle fit, and now she's great. I've also heard great things about the thick memory foam matrix half pads.
my horse has kissing spine (his back is essentially pinching him when he bends) but we treat it with therapy. i would check with your vet, see what is up asap, then check your saddle fit. no pads will do anything unless your saddle fits. some good SHOCK RESISTANT (that don't fix saddle fit and won't place getting it fit) include ogilvy, thinline, kavallerie, total saddle fit, and ecogold.
I would definitely recommend chiro and checking saddle fit. If he's still sore after that you may want to consider doing stretches with him and some ground exercises to build back muscle and strengthen his back(this so helped my horse with his back issues). If he's still sore after all that you may want to consider a vet check to make sure there's nothing really wrong with him. If he's just cold backed I would recommend back on track pads.