I would also be concerned for your physical safety once he realises that you are not taking him back. I would not allow him in your home and would only meet in a public/neutral space.
Also use the Grey Rock technique - don’t give anymore air time and oxygen to his nonsense. Just emotionally detach and withdraw with quiet dignity and carry on with your plans for a quiet life.
Try to emotionally protect yourself and swat any thoughts of him out of your mind and try to distract and replace with positive thoughts.
Expect his abuse to escalate massively once he realises there is no going back.
Expect the begging, pleading, crying histrionics once he realises that his few hours of Mr Fake Manipulative Nice didn’t reel you back in.
He might well stalk, harass, claim a crisis to draw you back in, threaten you and threaten suicide.
Be prepared and try to protect yourself from any of this. Do what you need to do - web doorbell, change locks, block his calls and texts - give him only an email that you will only engage with on baby/separation logistics and you will only read and reply to on a weekly basis.
Have police on speed dial if he steps over or threatens to step over the line even just the once. They take this v serious and will clip his wings or arrest him. If he threatens suicide call his GP or 999 - it’s a job for the professionals. He is a v dangerous character many of us know his character and MO when rejected. We have seen the patterns hundreds of times.
Tell your friends and family and his family immediately he says, does or threatens anything - sunlight is the best disinfectant.
Do not give any airtime to his woes or begging / discussions around separating. Just do this cleanly though the official channels.
You have done really well and taken some v important steps by realising this is v wrong, seeking advice from a solicitor, seeking advice here, getting him out of the house, seeking support from your friends and family. A v successful phase 1.
Phase 2 is the eruption when he realises you mean it. He will do anything to punish you - so you need all of your defences ready. This part is tough - he might wear you down - on average abused wives take the partners back 7 times before they finish it for good. In all of that time the abuse escalates and they and their children are repeatedly and deeply emotionally injured.
They all deeply regret taking them back.
I wish you the strength and resilience not to be this statistic.
You might also want to consider his capacity to care for your baby during access and the potential for risk due to his addiction and MH issues. Even if you have a grain of concern I would look the courts for supervised access only. This shouldn’t be an issue if he is already recorded and documented in the system (NHS).
He is truly a despicable vile individual. His MH issues and addictions do not excuse his abuse. They are separate issues he chooses to be abusive to you.
Might be hard to appreciate at this moment but you are in an infinitely better place than when you started this thread and once you face off his inevitable punishment grenades and have him in your rear view mirror - you and your babies life will continue to grow wonderfully together without him polluting it.