Woosey Dion hugs to you both.
All I can say is I grew up with an emotionally, physically, financially abusive step father. Me and my mam got the worst of it. He is now diagnosed as a paranoid schizophrenic but 30 odd years ago he was just a charming man who loved his family and was strict 😐.
I won't go into the ins and outs of it all. Think Jack Nicholson in The Shining if you want a mental image.
I was the oldest and also the only step so saw and remember lots of fucking awful things. My youngest sibling is 11 years younger than me, I was 17 when my mam finally left.
I went N C immediately with step father. It has caused issues with my siblings who just don't understand why I hate him so fucking much. I also hated my mum for a long time for not leaving sooner.
I will say though that as we all started having our own dcs and own grown up relationships our views have changed. Although he is still their dad my siblings now understand a little more.
I also chose not to tell them of some of the horrific things he did. They don't believe me anyway, it was years ago, his hatred was directed at me and my mum and he worships them so they will never experience or understand it.
My advice for what it is worth is that it isn't worth arguing over now. Let DS believe that you ruined his life Dion. He is a young adult. Not some sad 6 year old waiting for his daddy to pick him up. Sit him down when you are both calm. Apologise that his childhood wasn't Topsy and Tim perfect but shit happens. Other kids have it worse. You made your decision based on all of the families safety and happiness not just his.
Tell him if he can't accept that he should contact student services for some counselling to discuss it with someone trained to help him. But you can't change what has happened in the past and he is welcome to go to his fathers in the holidays rather than yours if he likes.
Hugs to you both. Abusive men cause problems even when you have left. Set of cunts the lot of them.