I have had a lot of success with equine massage. It gets all the knots worked out and frees up the horse to move again. I'd start there and then start a daily stretching program so he doesn't stiffen up again. If you do get him a massage, see if you can pick up a few pointers on how to do some techniques yourself. I learned enough to where I can identify knots and in most cases work them out myself. It really seems to help my horse a lot.
You just have to see your distance...you don't have to like it.