An out of the blue information release, for which there was no prior ground laying is a hell of a shock. When the shattering of your perception of a parent is part of that, it leaves room for a profound and enduring sense of loss.
Revelations about your parent's very very clay like feet can feel like an earthquake going off right under you. Some kids keep their feet during the tremor and cracks are easily repaired, some wobble hard go down to their knees but get up again with just light bruising not too long after.
Some fall down hard and struggle for a very long time to get back upright. Rising up in a landscape that can feel irrevocably changed. And not in a good way.
He knows you. If the only thing he could read from your tone, your language choices, your facial expressions and body language was concern for him, that will be part of the tool kit he can use to regain his equilibrium.
If there was any way he could have perceived anything that tinged the conversation with an element of less altruistic motivation, that will go in his personal tool box too. That is neither an accusation, nor a criticism. Humans are not robots and high tensions often lead to long standing emotional hangovers that can leak out, even from people who love and fundamentally want the best for their children.
Did you see the film Inside Out ? Specifically the formation of the islands that shape personality and what makes you.. you.
I can remember with clarity the memories that made 'islands" for myself and my siblings. And they aren't all shiny. Some of them are dark, twisted places formed after parental revelations about the other parent. As a sibling group of three we have had varying degrees of success in controlling the extent to which those "islands" expanded and built suburbs. Or went the way of Detroit, there, but essentially decaying, dying and becoming ghost towns.
Even with the best of intensions revelatory, shocking moments in time can generate those sorts of pivotal memories that have the power to shape us for decades, because they feature strongly in the decision making processes life demands of us all, as we go from child to adult.
Which is why in your position I would focus on getting quality support for him in place before it becomes apparent that he needs it. I don't live in the UK, so have no idea what the lay of the land is now, but would suggest school and the GP as a first stop off point to get information and guidance.
You didn't make his dad hit you. You didn't make his dad be the arse he is. You didn't create the context that placed a shocking revelation on the table as potentially necessary. You had no control over any of that, which is a horrible disempowering place to be. But you do have the power to control the extent to which impartial, quality support and informed guidance is accessible to your son, should he need it.
To some this will sound overegged. Like I said. Some kids wobble, but don't fall, or if they do get up again fairly fast with few bruises. If that is all somebody has ever known then they may not be aware of just how life-impacting an event like this can be for some of the kids, some of the time. There isn't always the recognition that events like this can create a kind of grief for a lost parent because your image of who they are shatters. I strongly suspect that a part of why some kids take so long to get back up, and do so covered in scars, is because they are left dealing long term with unacknowledged grief.
It is clear that you love your son and are fearful of the horrendously negative impact his father could have on his formation and future life chances. That points to a very motivated parent who is more than willing to help him. Which means he has a fighting chance of working through his destroyed image of his dad. With your guided and supplemented, emotional support cushioning him as he comes to terms with the loss of the father he thought he had.