At the age of 22, are you real sure he is not getting arthritic? When a horse is sore in both side of his body, they cannot limp and show that they are in pain when they move. It takes diagnostic tests to locate the reason a horse is not moving forward like it should. They frequently get foot pain or heel pain equally in both front feet. Both hocks can be sore -- and on and on. There are a hundred things that can cause pain that is not demonstrated by having a visible limp.
Before you invest in a bunch of x-rays, why don't you give a full level of Bute twice a day for 2 days, give him 2 grams the third morning and ride him hard on the 3rd day? If he rides differently with a pain killer in his system, you know you have a pain problem. THEN, you can start hunting the cause. It may be treatable or it just might be age catching up with him. You may get relief from different shoes or hoof angles. You may get relief by giving him Legend or Adequan shots or dietary supplements. This is where I would start. It is not fair to try to force him to work if he has pain issues.