Look, you don't have to answer this obviously, but I am dying to know because I am at a loss to understand any of what's gone on to lead up to your children being removed - why were they taken to live with your parents?
By your own admission the DV between you and your ex was a long, long time ago and it doesn't seem to be the reason why SS removed your children in recent months anyway.
He's been gone two years. He is apparently the only reason SS were crawling all over you in the first place.
You may have some PTSD relating to things in your past and you need to get support for that but you say you have never been seriously mentally ill to the point where you could not look after your children.
Yet they were taken from you a matter of months ago. And SS are saying there is no sign of improvement or change in the situation at the moment. 
By all means tell me to mind my own business but I just don't get it. At all.
And back to the original question/subject of your OP, of course you weren't asking how to get your children back and of course you don't have to justify why they aren't with you in the first place or divulge your whole backstory, but given you said it was 'all his fault' that you lost them in the first place and then you tell us he's having another child, naturally the obvious questions will be asked. We can't possibly just ignore the obvious and respond to the OP with anything meaningful without touching on why you find yourself at this point.
If he did anything monumentally terrible then SS will already be on the case with his new partner (as you have already said they are) and she may yet not get to keep that child unless she is prepared to sacrifice her relationship with him, so they don't necessarily get the happy ending that you are being denied.
And as someone upthread pointed out, he may have held his hands up to his past mistakes and jumped through all the right hoops to demonstrate to SS and HCPs etc that he is committed to working towards being a better partner and a better father in future.
Have SS told you you are not to move nearer to your children, and are your visits supervised as a condition of your access to them?