Stress could be the cause. My mum was going through a tough stressful time a couple of years ago and broke out in a rash, was very itchy and nothing seemed to fix it. Then she got help for what turned out to be depression and the rash went away (not saying your DH is depressed, just making the stressed out comparison!).
Obviously telling your DH to stress less is easier said than done - myself personally I found acupuncture to be amazing for stress when I was doing my PhD and going through fertility issues. Is that something he might be keen to try? Otherwise just trying to take some time out to relax, even if it's just an hour on the weekend for him to chill out, may be useful. And of course take care of yourself too if you are also stressed out!