Withdrawing application after recieving OWP and ETA

No, “to whom it may concern” is fine. There is no specific format. Write a formal letter requesting that the application be withdrawn and hand sign it. Submit the letter here: https://www.cic.gc.ca/english/contacts/web-form.asp

The withdrawal process is usually completed anywhere from a couple days to a few weeks. IRCC will let you know once the application has been withdrawn.

If he divorces you and lives with another continuously (with no breaks) for a year in a marriage like relationship, he can apply for PR through common law.