TBH all of this is making me seriously question DH
You made a mistake and he made a mistake - it what you do now that is going to define you both and the family that you have around you, both estranged and close.
My DH had a strained relationship with my oldest DSS for years, he is nearly 18 - TBH there are times that we have argued about it and I questioned whether I wanted to stay with him over it but.....
DH and oldest DSS mom slept together as a one night stand and were stupid and didn't use anything and DSS was conceived (21 at time). They remained friends and DH was at the birth and had DSS EW from a baby.
DH (a bit of a knobhead in his younger days) met someone else when DSS was 4 and they were together for a few months and low and behold, never learned from his mistakes and she got pregnant. Now actually whilst new GF was great with DSS to start with as soon as she had DSD she started rejecting him and it caused a breakdown between her and DH's relationship as basically she started to treat DSS like a second class citizen. DH could have left her but didn't because he didn't want to be in same situation with DSD. He had placed himself in a ridiculous situation and was not mature enough to deal with the situation and was trying to (badly) juggle the needs of both children and deal with an DSS mom who (fairly) hated new GF and GF who was resentful of his past and admitted she got pregnant because was jealous of relationship with DSS and hoped that once DSD came along he wouldn't be interested. This carried on for a couple of years and GF wanted new baby and DH refused saying they were straining as is without another baby. GF got pregnant again and DH left.
Now DH and I got together when GF was pregnant (again I was young and a bit green). By the time the newest baby youngest DSS was 1 we had moved in together, had all DSC EW along with my own DS. Things ticked along with the usual fall outs and make ups between DSD and DS mom and us (not eldest DSS mom as she was always fine with me).
When DH was 12 DSS mom and him had been struggling, she went on to have 3 other children and was not coping. It resulted in us receiving a call from school advising SS and police had been called because DSS had gone to school with strangulation marks around neck. Mom was cautioned by police and DSS was placed in our care and had access to his mom. We were making plans with SS for DSS to live with us (all this was as he was just about to turn 13). One day we received a call from SS advising that DH did not have PR for DSS and mom had changed her mind and wanted him back and did not want us to see him any more. He was removed from us that day. In meantime ex GF was very bitter that DSS had moved in with us stopped access to DSD and DSS.
DH was now in situation where he was having to go to court to try and see all his kids. SS had advised that DSS refused to see them and said he wanted to live with mom and didn't want to see dad as dad said his mom was a bad mom (never did to my knowledge) sol was advising that DSS was old enough to make decision despite SS being involved and courts take his feelings in to account. He went to court for other 2 and still got EW access but didn't pursue with DSS.
He had made it clear through letters passed through family support that DSS could see him anytime and we lived 10 mins walk so he was old enough.
Again within 12 months DSS had been placed with another family member (this time nearly 14), SS did not tell us because mom and requested not and so had DSS.
At 16 DSS admitted that he loved his mom and was worried that if he saw dad or lived with dad his mom would reject him and it would make things worse at home.
He felt DH and fought for others but not for him despite him refusing help and contact several times.
Now he is nearly 18 he still gets angry sometimes although we see him now he but admits that he played a part in breakdown but mainly he only comes round he wants money( partly due to age and partly due to past).
I feel that we let him down and DH feels that he should have done more but everything was happening at once and it was handled badly by everyone.
I suppose my point is that things happen and some things we have to stand up and take responsibility for. Over years DH, ex GF and DSS mom have discussed things (at different times) and all admit blame at different points for different things. None of us can absolve our selves from the responsibility that we had or should have taken for all the kids involved in the drama that was our lives. At various time we let our kids down badly in different ways inc my DS who got caught in cross fire and went from all kids being around him to none to some again.
On the face of the story my DH should have been dumped for being spineless, not using contraception, not fighting harder and it goes on and on BUT, I love him and I know he loves me - yes he has made stupid choices and mistakes but he has also fought for them differently at different times and not enough at others but situations are never truly black and white so sometimes you have to just let go of the past and focus on the future and what you can make out of it for everyone involved.
Sorry very long but hopefully you go a bit easier on yourself and your situation as recriminations will eat away at you and not help anyone.