I've been lurking, because I saw your OP and thought "As this goes on, she's going to reveal more and more examples of him being an abuser", and, as my advice is always v blunt, I didn't want to barge in commenting on very little info.
Brave, you should name your child Massive Red Flags, because everything you write about this man is a massive red flag. Classic, classic signs, actons and words of an abuser. He's not even original. Abuse very often worsens or becomes more apparent during pregnancy or after childbirth.
He seems to be using you to work through his anger at his ex and his displeasure at the name choice of his other child (I bet no name compromise would've been good enough then either). This is a metaphor for how he, like all abusers, treat their partners - you are merely an object, a conduit, through which he gets what he wants, takes his temper out on, and gets revenge on for everything that pisses him off or disappoints or upset him in life. Such men are deeply inadequate and cannot change; they are emotionally stunted at a very young level and rarely if ever improve or change, although they might appear to change temporarily to suck you back in. This prize catch doesn't even care about you enough to make bullshit promises or try to appear to change, that's how little he respects you or gives a shit about your feelings and needs. Not that pretending to care or change is positive, I hasten to add.
Your talk of 51% positive being enough is very upsetting, but also very telling. As a child, you would've had to make sense of the Daddy you loved behaving badly, and you know doubt made it okay for yourself by telling yourself that so long as he is nice just over half the time, then everything is okay. I bet your Mum said similar stuff.
If you value yourself so low that only 51% is enough (and I bet a lot of that isn't actually great, just 'not bad'), then no-one else will value you more. Many women have partners who are okay or even lovely 80% of the time, but who beat and rape them and treat them like shit the other 20%. Does that make it worth staying? And do you know how much percent bad behaviour you should accept for a relationship to be worthwhile? ZERO.
Stay with this man, and the family atmosphere will train a son to be a future abuser and a daughter to be a future abuse victim. People think children don't notice their mothers being treated badly, but they dol even if itis unconscious. Look at you - your father treated your mother like crap, even though you say you didn't totally realise, and yet here you are, a victim of abuse. If you have a daughter, the name issue might be avoided, but she will grow up to be treated like shit, just like you are being.
On a last and practical note, as you are not married then actually and legally, your child should be given YOUR surname. It is a permissible legal courtesy that unmarried mothers can give their child the father's surname or double-barrell it. It is outrageous to demand a certain first name without any compromise, but it is a clear sign of the disdain he has for you that he won't even 'let' the child have your surname as a middle name when legally it should have the same surname as you!
Admitting you have made a mistake and leaving someone is never wrong. Staying with someone like this is always wrong. Just the way he said 'maybe' last night proves you shouldn't be with him, never mind the rest.