You may have to open an account for yourself if the account was in his name. Customer care will do nothing for you if its not in your name. You may be better off getting your own account so this doesnt happen in the future. I understand not everyone can get post paid, but pre paid is just as good and builds trust with Koodo.
If your ex is the account holder, then you will need him to do a transfer of ownership of the line to you. You would need to pass a credit check for this. Then, your number, your plan, and any balance owing on the account will become your responsibility.
If he's cancelled your line, your number is gone and you'll need to get a new one.