I cut out sugar and caffeine as the spikes I got from them made me crash hard, I felt more on a level then up and down constantly with the exhaustion. (It didnt cure me or anything but I felt more in control which certainly helped).
White rice etc can also do this, google things that can make your blood sugar spike and try changing those, i.e white bread to wholemeal / white rice to brown etc.
I don't really agree with gluten free but it may help, it may not, don't cut it out for no reason though (does he have bowel issues?).
Put more anti inflammitories into the diet, like turmeric, just put it in food randomly, it doesn't taste of much but it's good! Anything anti inflammitory will be good for him.
If he doesn't yet, get him to use epsom salts, they can help a lot with muscle aches.
I don't know what your husbands dietry issues are if any, but a lot of people with m.e have really bad reactions to some foods, bowel or exhaustion or otherwise, nothing works for everyone but hopefully with some trial and error something will help a bit.
Also, it really made my night that you're making such an effort, too many of us are "abandoned" by friends and family so it's very lovely to see this post.