For me the best treatment for a horses back is always Chiropraxis and Eletroacupunture... In my own experience riding and making It on the horses. The both produce a Very good pain less action, improvement of biomechanical, and enhance the muscles in the Body.
There may be several causes to backache, so a vet, especially one that specialises in orthopaedic matters, should be your first point of call.
Ruling out congenital or metabolic issues, he may recommend osteopathic treatment, which in my experience can be very beneficial, but it really depends on the ability of the osteopath. You may be lucky and find a vet who is also an osteopath.
Furthermore, I would recommend a visit by a saddler to ensure proper saddle fit.
Finally, work needs to be progressive and in keeping with your horse's condition; for example, warm him up at the lunge before saddling up, and avoid sitting the trot.
Your first port of call should be your vet to find out the cause of the backache. From there, they could give you which local therapist they think would help the most for your particular horse.