Some Vets will tell the owner of a dog with back pain that the dog has a disc problem. Well to specify that situation an MRI needs to be done which is the only test that can assure the owner of a disc problem. This test is very expensive. Very commonly the dog will have subluxation [ bones irritating nerves due to improper movement ]. A Disc problem may exist but is not usually the most common cause.
Having a skilled board certified animal chiropractor evaluate your dog is the best option after a proper vet evaluation minus the MRI. If the chiropractor also feels it may be a disc problem they will suggest an MRI as well, but if the dog is older then chiropractic adjustments along with PT will many times help this dog. The dog will have to be put under for an MRI and this in of itself can kill the dog if he is older.
A trial course of chiropractic is your best option. The adjustments take into account the potential for disc problems so that if it exists no aggravation will occur. Most Vets have no training in chiropractic so they think that the chiropractor will twist the spine and injure the dog, this is simply not true. I have been in practice for 30 years and have adjusted dogs with disc problems as well as people and have extensive education in chiropractic care.