I would make sure he gets checked for B12 and D deficiencies at the minmium. Ferritin is a storage protein for iron. If it is low, it could mean his iron needs are currently being met by robbing from storage but the storage is not being replenished. I find it interesting that his fatigue set in during the winter months. If you live in a northern latitude or where there is a lot of cloudy weather during t he winter months, your husband could be experiencing low vitamin D levels. Even aside from geography and climate considerations, people who spend most of their time indoors are often vitamin D deficient. Low vitamin D levels is a chronic problem in developed countries where people's livelihoods keep them indoors during the day.

Yes he has had recent blood work.  Other than low ferritin everything else was normal 

@LizzieF, if you will be seeking repeat blood antibody testing after undergoing a more rigorous gluten challenge, make sure your physician orders the "total IGA" test to check for IGA deficiency. If you are IGA deficient, all other IGA test scores will be artificially low and this can result in false negatives. Can you list the blood antibody tests that were already done, the scores along with the reference ranges?

@LizzieF, The anemia which can be caused by low iron can impede antibody production.   NCGS can be a precursor of Celiac Disease, especially in those who already carry Celiac genes.   Celiac is the only disease where you have to self harm to get a diagnosis.   Do talk to your doctor about the updated gluten challenge requirements and a repeat endoscopy.

Welcome to the forum, @LizzieF! Yes, I also question whether or not your exposure to gluten before either the endoscopy/biopsy or the antibody testing was long enough or intense enough to render valid test results. Guidelines for the "gluten challenge" have recently been revised to recommend more intense exposure: at least 10g of gluten (about the amount in 4-6 slices of wheat bread) daily for at least two weeks leading up to the day of the blood draw or the procedure. But frankly, I would certainly give it twice that long to be sure. It is also very possible you are dealing with NCGS (Non Celiac Gluten Sensitivity) instead of celiac disease. They share many of the same symptoms but NCGS does not damage the villous lining of the small bowel. Therefore, not antibodies are produced and nothing will show up on a biopsy either. There is no test for NCGS and it is 10x more common than celiac disease. Some experts believe NCGS can be a precursor to celiac disease. Celiac diseases must first be ruled out to arrive at a dx of NCGS.