Get a qualified Bak Man (sorry don't know accurate profession name
) out to check his back over. A great deal of lameness can be caused from having a sore back as the horse compensates for the pain. Get his back checked over, then get a qualified saddler to properly fit your saddle, and get him to work with the Back Man if possible, and see where that leaves you. It could be worth getting the Back Man out when the horse is sore, and after a week or so of rest when he isn't so sore and see what conclusions he can draw from this also.