Hey there, I'm currently going through this right now, just waiting to get my COPR in the mail. When your husband gets DM, he will get an email known as PPR (passport request). That name is a little misleading because since he's from the US, he does not need to submit his physical passport - he only needs to submit a photocopy. In the PPR email they will request four things from you to be sent to Ottawa. The passport photocopy, 2 new photos exactly like the ones you submitted with the app, a prepaid envelope for them to send back your COPR in, and a form called Annex A that will be attached to the email (very simple, just has you write the address where the COPR needs to be mailed). I am also in Canada and they provide you with specific instructions how to send if you are inside Canada vs. outside. You will send everything via Canada Post and get your prepaid envelope from Canada Post as well.
It takes them about 2-3 weeks to send the COPR after you send in your documents, and when he receives that, he can land whenever he wants. Good luck!