First of all congrat on being und. actually tx play with our metabolism and some times with thyroid. Check thryroid performance.
Peg – Can you supply a little more information? Has he been eating and losing weight or has he stopped eating because of side effects?
If it's the latter there are a few practical steps that can help: cold food vs. hot, many small meals vs. a few big meals. Although it's damn difficult, he needs to try to do everything possible to keep his weight up. I keep a bag of Paul Newman's chocolate alphabet cookies in my office at all times and eat them in large quantities around 10:30 in the morning and 3:00 in the afternoon. If you're not feeling well, it's not worth worrying too much about nutrition. Any calories that he can tolerate are better than none at all. For example, I eat frozen waffles with maple syrup just about every day for breakfast.
The other thing that seems a little counter intuitive is to try a little exercise. When I was at my sickest from side effects, I lost 10lbs in two weeks. A certain amount of that was "muscle wasting" because I was dragging around. Exercise, even if it is very modest, can help a bit with muscle mass. It might also make him feel better.
Good luck. It's great that he's been a fast responder. 40lbs sure is a hell of a lot of weight. Hope he can stabilize for the rest of treatment.