Hi sorry to hear of your frustration. All of the signs you have mentioned point to lymes. The rushing at the canter, not wanting to go forward, bute and previcox having minimal effects ect. The injections you have done so far will help him in the long run. I would have him tested. Make sure you use the test that will tell you what the actual titer on your horse is. It cost more money but well worth it. If this is negative then perhaps look at kissing spine or epm. Best of luck to you and your horse.