As someone who has supported a husband through chronic anxiety, merging into depression, for a decade, there are some simple things you can do, that will make a real difference if it is depression, and will probably help even if it's not. They probably won't completely solve the problem, but they help us keep things at a manageable level (at least until major external stressors come along. Thanks, mystery illnesses!).
1. SLEEP. Uninterrupted, restful sleep. If either of you aren't getting it, start there.
2. Exercise. Really helps. It really really does. And it makes getting 1 easier! If you "don't have time" look at doing High Intensity Interval Training. 10 minutes will get you panting and sweating, which is of course the point of exercise. Fitness Blender does a fantastic range of guided videos (free online) that can help. No excuses for not exercising - if you have bad knees, do push ups. If you have bad joints, swim. Or if you can't take intense exercise, do longer stretches of low intensity exercise. Other than sleep, regular exercise makes the biggest impact on DH's moods.
3. Social circle. Belonging is important!
4. Nutrition. You don't have to be paleo, but THREE slices of cake for breakfast? Every day? More veggies, less process.
5. Remove external stressors. I put this last, because it doesn't sound like the problem in your situation, and also, because if you could easily remove them, I figure you already would have.
On another note, if it is the loss of feeling he is an expert that is the issue, is there something else he is an expert at that he can focus more on? A hobby? Volunteering? Watching sports and critiquing the plays?