Disagree, he doesn't want you back, he wants CONTROL back again. he wants you under his thumb and beholden to his every word.
Well that Genie is well and truly out of the bottle, you can't ever go back to sucking it up for him and being told what to do think and say anymore, it'll never happen.
Agree with barbie, BE A WALL, don't let a glimmer of hope get thorough to him, cut his arguments off every single time, refuse to argue, refuse to engage. he doesn't want to care for his DC, fine, do it yourself, but make sure he knows that he is failing you and your DC by refusing to do his share and this is one of the HUGE number of reasons why you are ending the relationship, because YOU deserve better and so do your DC. A good stepdad will help them immensely in life.
If he kicks off at anything say "I can tell you are not able to have a rational adult conversation so I'll leave it there". Leave the room, go out, don't even tell him where you are going, leave the phone.
He shouts at you on the phone, hang up. Emails you a bunch of shite, just ignore and create a filter on the inbox to put all his emails straight into a filter marked TWAT.
zero tolerance, zero engagement, zero feeding his sad little ego