Just as a follow-up in case anyone else has this question.
The USCIS at the embassy determined that his AOS was still pending (!!!!!) after nearly 2 years at the Memphis sub-office. They said that as long as I cancelled his application, he should be okay to enter the US as a visitor (visa-waiver).
So, we Fed-Exed a simple letter stating that we wanted to cancel the AOS effective immediately and took a copy of that letter, a copy of the email from the embassy, and some other things (bank statements, etc).
The very nice immigration officer at Memphis looked at the email, the letter, and looked a bit confused but we explained everything and assured him that it was a visit, not a move and he stamped DH's visa-waiver card. No problems.
***This is by no means legal advice nor is it intended to replace legal or consular advice. This is merely an account of my own personal experience and we happened to have a particularly straightforward case.****