He doesn't even know

Long story short I havent heard from my ex bf since middle of August. Last time we talked it was basically him telling me if he couldn't have me he didnt want our babies. I was about 19wks pregnant. He had multiple gfs while we dated even got one after I was pregnant. Basically he didn't want to be a dad. Wasn't planning to change diapers, feed, bathe, etc. Even suggested we give one of the twins up when we found out they were twins. Fast forward I gave birth and unfortunately one passed away. Still haven't heard from him and I as far as I know he doesnt know they were born or anything. I'm planning to get a lawyer to have him acknowledged as my babys father and have him sign rights over. Anyone been thru anything similar that could give some insight or advice.

I should edit to add that hes not on the birth certificate but I want it done so that he cant ever come back and try to get rights to her