sex with ex

My ex and I broke up while I was pregnant with our third baby. My son is now 6 months old and I had a moment of weakeness and slept with him. I'm not on any contraception and am getting paranoid I'm going to get pregnant. 
If I do get pregnant I won't be able to go through with an abortion. It would just kill me to do that. But my family hate him and after everything to have another baby with him would just be so weird!!! 
Tell me what you guys would do? I need some reassurance... even from people who have been through this as well.