It took me 4 times to pass - I got really nervous as well. I only had a 3 week gap between my third and fourth test (if he fails this one, try and book the next one for as soon as possible), which meant I didn't have too long to work myself up about it. And the day before it, I had a nice day at home - a bbq with the family. So my advice would be do something nice the day before. I found kalms never really helped me, but bachs rescue remedy does help.
(I found I was so nervous I couldn't even talk to the examiner, I found it easier to do it in silence...so chatting to the examiner wasn't the best advice in my case!) And try to get your husband in the mindset, that even if he does fail it this time, he'll just do it again. And again if need be...you get the point! Hope he passes - good luck!