I am an equine therapist and I will tell you that trying to stretch a muscle when there is a problem is not always a good idea. You should do some massage first to loosen up the areas and help release any restrictions he may have.
Besides the actual back muscles, look for the junction along the top of his hip where the back muscles join the gluteals. You will often feel a tense area of tissue about halfway between the bone on the top of the pelvis (tuber sacral) and the bone on the side of the pelvis (tuber coxae).
If you can massage this and help loosen it up and then massage the back, it should help relieve the back. Most of the time though, back problems don't start in the back - unless you have a poorly fitting saddle, etc.
I have some pages on my website that you might find helpful. Equine Massage Therapy Basic Movements Equine Stretching Exercises Equine Chiropractic Information
They are about Equine massage techniques, stretching exercises, and some things you might want to know before using equine chiropractic.
I hope this helps.