Thank you all for the answers, we got the vet and she said hat he trots asymmetrically and that's where the pain comes, the only thing we can do his work on strengthening his back, can you recommend any exercises? his doing lunge work with poles twice a week for now and I've been riding the other four days
Chiropractor’s work magic! my mare was soooo tense, and still is, but a checkup and adjustment from the chiropractor every three months keeps her feeling good! my chiropractor pinpoints the area or areas of tenseness, adjusts the horse, then if needed does some acupuncture, and then assigns us with stretches to do on a regular basis before rides to help the tenseness stay away for as long as possible. I absolutely swear by the chiropractor, he or she will usually give you everything you need to keep your horse in tip top condition. If he thinks you need to get a saddle fitter out, he or she will tell you. hope you figure it out!
The first thing to do is to find the reason the horse has back pain and whether it's definitely in the back. The obvious go to is the saddle first. If the saddlers, farrier and dentist is happy with the horse, then your next port of call is the vet. There's a good chance your chiro/physio will ask for a vet's permission anyway, so it's usually easiest just to get the vet out before them.
Obviously I would recommend a vet look at him but what we have done with our guys is massage therapy or Chiropractic work. When that doesn't work or you need more assistance we use methocarbamol or miso therapy which has worked really great for horses with spasms some of our older guys we put on previcox. Good luck! Make sure your saddle fits too :-)