iMessage sharing and changing AppleID
There is no feature to allow someone to sign in to a second Apple ID, so unless he guessed your new Apple ID and new password, he's not receiving your messages.

You could enable two factor authentication to ensure no one easily signs in as you. There are forensic and legal ways to read people's messages - but they are generally complicated or costly.

Most of this is explained at https://appleid.apple.com and if there is something specific you don't get - you can ask a new question more narrowly focusing on one aspect of privacy since the site works best with one question per question.

In summary, unless your husband (or ex) has installed key logging software on a computer, you should be able to prevent them from receiving any messages for you by changing your Apple ID password and making sure the phone company has a separate account for your devices.