I vaguely recall that I got back to the UK with the return half of my Visa Waiveronce, and being (needlessly as it turned out) worried about it. I'm sure it's quite a frequent occurrence, and that the cards get lost between return checkin desk and authorities from time to time too. There's no exit stamps from the USA - no mechanism for them to provide one
I've found that travelling to the USA on Visa Waiver, British Passport, American wife is easy. Go through immigration together and the'll be far more friendly to your husband than they are to "unaccompanied aliens". (I'm guessing he's British, and can check all the correct boxes on the form - he's not, for example, left the USA claiming diplomatic immunity in the past. Also guessing that you're going for holiday / to visit family and returning to the UK).
P.S. If you don't get much/enough of an answer in the quiet "Travel and Transport" section you might like to try "Visas" too. But you'll be fine.