Oh love, that's a lot of issues with a large chunk of physical and emotional risk involved.
From (distant) memory, many years ago once the child in question got to a certain age (think it was 11-ish) their wishes were taken into account with regards to contact. So from my (lay, out of date and potentially wrong) perspective I'm not convinced legalities would cause you too many issues,
However even if that is the case, I think the larger issue is how to manage communicating vastly reduced/supervised contact, or no contact given your ex's potential angry reaction to it.
Again, I think school might be your best first port of call. They may be able to point you to (or collaborate in contacting) internal or external agencies who work with children whose parents are putting them at risk. Having some kind of formal support leading the push against wholly unacceptable parenting choices might function as a buffer between you/your son and his father's anger.
Alternatively, I found the NSPCC to be amazingly helpful when I called them (although this was back in the 80s, and things may have changed). They were more than happy to function as a buffer in terms of underlining which parental choices were going to have to be off the table, or else. And I found them very child centred at a time where it felt like adult needs, wants and preferences were sucking up all the oxygen in the room.
I don't believe that leaving him at risk, which is what his father is doing, is a solution. And as much cushioning between you/your son and his dad as possible might prove necessary while this gets worked through.
Although it reads as a pretty black and white picture of an inadequate parent who should not be allowed to put his child at risk, your son might still need ongoing support (or at least the offer of it if in the future he feels it is needed) to deal with any emotional fall out from full, or semi estrangement.
Life would be so much tidier if kids could just stop loving, or wanting to be loved by, a parent who can't/won't be what they need to be. But for a good chunk of kids it ends up messier than that.
i'm so sorry you and your son are going through this. I wish empathy were a more useful tool that could used to create a solution, but it's not. All the same ...