Whatever behaviors (and instability, inconsistency) you saw in the marriage will be there in the divorce, and after.
The past patterns predict the future, more or less.
You probably should have some solid boundaries regarding parenting. Despite his insistence for added parenting time, you know the history and it's unlikely he will parent more than he has been doing. So one boundary is to consistently set firm limits to the parenting. And frankly, without a court order in place specifying otherwise, the parent in possession typically gets his or her way. In the early years of my separation and divorce, anytime the police were called they pleaded with us to work it out but then left saying, "Fix it in court." They would take action to resolve the immediate incident but that was about it.
Odds are that if he does get additional time he will be (1) a frequent no-show, (2) return the child early or (3) find some relative, neighbor or friend to be a child sitter.
Court may be inclined to give him a 'typical' dad schedule, alternate weekends and an evening or overnight in between. Courts like alternate weekends, it gives each parent a full weekend to be with the kids and also a weekend off from parenting. Unless he has a history of child abuse, neglect or endangerment then that's the most likely legal outcome. However, some states are starting to default or lean toward 50/50 so you'll need legal advice from a family law attorney (or a few inexpensive consultations) to get a feel for what your local court is likely to do.
If you two do end up having alternate weekends, dad does not have to have a full 3 day weekend. I started with a full 72 hours Friday pm to Monday pm while my son was too young for school. Once he started school, weekends were from Friday after school to Monday arrival at school. Of course I was a relatively normal dad. Some mothers here had even shorter weekends for messed-up dad. Maybe dad's time started on Saturday. Or the child was returned Sunday afternoon so the kid was able to get to bed and be ready for school Monday morning. You get the idea, ponder what would work best for you and the kids and consistently advocate for a schedule that would be most likely to work.
You may find that he ends up like a lot of dads with BPD, where he wants to win in court but then falls down on the job in reality, where life is not quite as filled with winning.
I am curious... .why the sudden desire to end the marriage? Does he want to work elsewhere? Did he find a fascinating replacement for you? Is he thinking that if he has more time in a schedule then he will pay less child support? (Frankly, in my court the calcs always came out very similar, regardless whether more time or less time, the amount was determined more by the relative income levels than the parenting time.)
You mentioned having a child changed the family dynamic. That's what happened to me too. Over the years my spouse was increasingly uneasy and I thought having a child would make her happy to see life's discovery through a child's eyes. Not to be, she relived her childhood fears through him. I learned the hard way that while a child is a blessing it doesn't fix a troubled marriage. So when he was getting close to the age her childhood abuser came into her life, she started getting antsy, unreasonable and paranoid. The marriage imploded and I could do nothing to divert the slow moving train wreck.
