My husband (age 28) got down to 125 pounds 2 years ago and I thought he was going to die. His lower back and all his joints hurt all the time. He had severe diarrhea and stomache problems which mimicked crohn's, arthritis, and several other diseases. Not sure if this is similar to what your husband is going through...but wanted to share our outcome.
We had been to MANY doctors and everyone had a different theory and gave him different prescriptions. None of them worked and he got sicker.
We went to a naturepath and was told he had a severe dairy intolerance. This sounded too simple, but he got off dairy and is now 165 pounds and pain free today!
I just would love to be able to help anyone who is going through what we did.
Would it hurt to try an elimination diet? Good luck and I pray that you find your answer.