I was recently notified that my ex boyfriend signed into iMessage on his laptop using my email address. I have since then changed my Apple ID password and was immediately asked to enter my new password into iCloud. If I changed the password, will it automatically sign out on his computer as well? And, if so, is he still able to use my email for iMessage?
Changing the password should be sufficient. You can see which devices are currently logged in with your Apple ID by following the instructions provided in this Apple support article: Check your Apple ID device list to see where you're signed in
You can also remove devices from your Apple ID profile by clicking the Remove button that appears in the device information box after you've clicked on one of the devices.
As an added step, I would highly recommend turning on two-factor authentication after you get everything squared away. You can read more about two-factor authentication and how to enable it in the following Apple support article: Two-factor authentication for Apple ID
If you change the password, he will can't login iMessages with your Apple ID. When he open it and want to launch iMessage, it will tell him the password is not correct. He can't use it any more.