If your husband is up for it he might start a food journal. That is what I did when I began lifting this winter. ( I took a short break from weight lifting for awhile.) My thinking was to see if any patterns showed up with weight loss and gain concerning foods that I ate. For each meal I would record what I was eating and every so often would record my weight.
I guess to tell my story if it might give him an idea - slowly, over months it appears I might have found a trend with the food journal. It is hard to say for sure, but seems that a meat or meat(s) cause weight loss and an upset gut. Nothing has been pinned down, but of late I've found myself only eating fish for protein, with some cheese from time to time. As a result weight is up, at 185lbs this evening. I started around 170lbs when I began the food journal early January. (I think this evening was an aberration, I'm probably closer to 180lbs. I was around 175lbs when i started the fish diet though so who knows.)
I'm avoiding other foods too, so I should not just highlight meats. There might be a spice, or something else problematic. Grains cause my gut issues leading to weight loss. That I have known for a long time. Regardless, details are yet to be worked out but most importantly my gut is healthier on the fish diet. It hasn't been this well in years. I've stopped the D. (The taste buds are not very happy about all the fish eating though!)
Hope the food journal idea might be of help, and that your husband finds some relief.