Would contacting a Child Custody solicitor or service (such as CAB) help you at all OP?
Have you any evidence of his going AWOL previously with not a care in the world until he came back to you begging for a second chance? If he were so invested why did he leave it until your DC was four months old?
Before he decided to leave again (and any idiot who thinks a successful relationship doesn’t involve “trying” is an absolute pillock) what was his parenting like? Was he fully on board in looking after your DC and do his fair share (honestly?) would you feel confident he could care for DC’s needs for one day alone? These are all the questions my friend had to consider when she and her ex split and he (after being a waster and a ‘Disney Dad’ before they even separated!) demanded 50/50 if not full custody.
Is he on DC’s birth certificate? I would imagine not if he didn’t turn up until they were 4 months old. This can be important when adding to your case if you go the legal route (which I would recommend).
Any abusive/manipulative texts need to be saved and try your best to remain level headed should you choose to respond to him at any point. If he badgers you stick to a standard dispassionate response.
I’m not saying cut him off or anything drastic, but you are and have always been primary carer for your DC. You have been there from conception to now (and will continue to be there) you haven’t swanned in and out of their life, so unless you can think of a reason why 50/50 with him isn’t such a bad idea (I only have a window into the situation but I’d say he’s already proven to be unreliable at best, at worst, you’ve said he’s aggressive and a manipulator) try not to let him bully you into anything.
I understand, as best I can, how drained and emotionally battered you must feel - but I urge you as one internet stranger to another, to not roll over. Seek legal advice as soon as you can and get the ball rolling if necessary on a childcare solution that benefits your DC. Try to keep them as your priority when it comes to the agreement that is set in place.
I’m so sorry you’re going through this @Thistlelair I wish you all the best. PM if you feel like a rant, I can’t offer much in the way of advice, but I can listen (well, read in this context!) 