If the doctors say there is nothing wrong with his back: that it is muscular, then read Dr Sarno’s latest book “The Divided Mind”.
Following his book has cured me of 95% of my long-term RSI come Fibromyalgia (which included excruciating back pain) in under a month. I have got my life back, and am too lazy to heal myself of the minuscule trace of pain that remains. I’m sorry to say this, but ignore the advice about beds, bending, stretching, acupuncture, elastic back supports, and in my case also chiropractors, rheumatolgists, osteopaths, masseuses, homoeopathy, different diets, drugs, exercise and many more: they might give some short-term release, but the problem is different – your husband probably has a strong health body.
He probably has emotions that are threatening to burst out.
If your husband is at all a people-pleaser or a perfectionist, or if for some reason he has trouble feeling and expressing his “negative” emotions now, or in the past, then he must read Dr Sarno.
(Although in my opinion, an ideologically compatible and better book on the subject is Scott Brady and William Proctor’s “Pain Free for Life”. Although the cover art could do with a little marketing skill.)
Good luck. The brain fights like crazy to keep its strategy of distraction.