@Longstockin you've had some really harsh and honest things said to you here (along with a lot of judgment).
It sounds like you've had a really traumatic time yourself and coped with a lot.
I can see that you feel your partner has made massive changes and you hope things can be easier and calmer now but I fear for you and I fear for your children.
It does sound like he's made a lot of changes but it's very early days and generally violent men revert to that behaviour unless they're had a serious amount of help and support to change.
Pregnancy and childbirth is a really risky time in DV relationships and I worry that things could escalate again. You'll both be tired, he won't be your focus. The healthiest relationship is heavily tested by a new baby.
Your son has experienced this domestic abuse, he will have been frightened and traumatised by this and now he is expected to live with the man that caused this.
I imagine he will be frightened about this happening again, maybe walking on eggshells, not wanting to cause any arguments. The fear doesn't stop when the violence does and your son is being expected to live with an abuser.
This will be further traumatising him and it's absolutely not ok. You are continuing to fail to protect him.
Your baby is at risk too. They too could well experience violence and abuse in their childhood.
You could have social services involved again, questioning why you are potentially exposing another child to this mess.
You are not the cause and you are not responsible for his behaviour but you do have a choice about whether you stay.
There is no relationship with his family and you don't see his children.
There is so much here that is against this relationship standing any chance of surviving.
It sounds like you are isolated. What's the deal with your own family? Do you have parents, siblings, friends that are yours and not his?
My advice would be for you to make plans to leave before your baby is born.
Get help from DV orgs and housing.
Get out and get prepared for your baby in peace and safety.
Please believe me that none of what I have said above is meant with anything but kindness and understanding. I am not judging or blaming you but I am placing the responsibility to break this chain on you. You have to act.