OK ... you may want to seek local professional help getting this mess corrected.
If you were still married on 12/31/18 then you two could only file the 2018 return jointly OR married filing separately OR as Head of Household if either of you qualify. If he filed separately and he owes then that would make sense that he wants to amend to file jointly. This situation should have been discussed in the divorce proceedings. If it was not you have no obligation to file a joint return unless you choose to do so ... talk to a local professional to get educated on your rights and options.
