I completely agree with the above poster. Find another Dr. Your husband could be having gallbladder problems. I'm surprised they did'nt do any testing for that. Actually, several different things could be going on. I really hope he sees another Dr that will find out what's going on.
The best advice that I can give is, find another doctor! Chronic pain and fatigue, when the standard causes have been eliminated, can be incredibily frustrating, both for the patient and the doctor. Unfortunately, as you say, some doctors stop trying when their usual tests do not reveal the cause.
At a minimum, seek a pain specialist. Such a doctor may have no better luck in finding the cause, but at least your husband's pain might be better controlled.
In addition, if there is a good medical center in your area, or a practice that employs a variety of specialists, try making an appointment there. Sometimes, your doctor may not want to admit when he is stumped. If the practice has a number of types of specialists, those doctors may be more inclined to seek a consultation from their peers.
Best of luck!