You have been put in the position to choose between a romantic/sexual relationship with this man and your child once, and you chose your child but then went back on that at the final hour. Now you're in the position to choose between a romantic/sexual relationship with this man and possible 2 children. Please, please leave this man. I know it can be hard to and I am sure this is made harder by still seeing each other and co parenting. But I would say in this situation your best policy is to come clean to everyone. Be really honest with everyone involved. Tell him that you regret sleeping with him and want to return co parenting (if that's what you want) , tell SS what's happened and ask them what better support you can get to not fall back under his spell again (freedom programme, an IDVA, counselling, groups, whatever they can offer you). Also take some initiative yourself. Read as much as you can about abuse, the impacts on children, trauma bonding, narcissism, etc. Arm yourself with all the facts.
As regards whether to have an abortion or not, that is up to you. Only you know you're true feelings on this. Don't feel bullied either way. You have a short bit of time to do some real soul searching. Yes there are economic struggles and single parenthood is hard and it's not ideal to be Bringing a child into this situation for various reasons, who their dad is (abusive) and with social services hanging over you like a heavy storm cloud.
But I have been in a similar situation and I chose to keep my baby. I moved to a refuge and I got every piece of help on offer. I've since been building my career, building a new life, had therapy, and I am grateful that I made the decision to keep my child every day. I am also so grateful to be able to have made a choice. To have really explored my options, because for me choosing not to have an abortion it was a big decision that day and I made it on the understanding that I would never, ever put my kids in the situation to have DV in their home again, that I was choosing my kids over any and every potential relationship, possibly until they all left home! For me it was a turning point where I took the steering wheel back in my life and decided to woman up, take responsibility for my whole situation even if it wasn't all my fault, step out of the drama triangle and become a circuit breaker for the pattern of abuse and generational trauma.
There is no world in which you get to keep any children and an abusive partner in your life. Maybe for a short time, but in the end if you choose the man you will lose the child. Every time. Don't you deserve to have a happy life? Doesn't your child? And even if you don't keep this baby, might you want another one some day? Don't limit yourself because of an abusive arsehole. You deserve better, and I believe you can do it. We all only get so many chances in life to get things right, you know that this relationship is wrong so please throw out the rubbish and prioritise yourself and your child over this piece of trash.