Hi Ninjacat, I'm so sorry you're having to deal with this . How old is your DS?
As far as I'm aware, you are within your rights to prevent him being forced into a situation where he is being threatened, but I'd get some legal advice ASAP if I were you.
I think you are going to have to take the responsibility for this off your son's shoulders, and tell your ex that he will have to visit him at YOUR home for the foreseeable future. You cannot ask a child to spend his weekend with a bunch of strangers, one of whom is a teenager threatening to stab him! . What delightful folk they sound!.
Your DS will naturally feel very guilty about not wanting to visit his Dad which is why you need to make this decision for him. His relationship is with his father, not his Dad's new DP and DCs. It's unbelievably selfish of your ex to have sprung this situation on your DS!
A cautionary tale. MY DS has always stayed over at his Dad's 2-3 times per week since he was 4. When he was 7, my ex started seeing a woman whom he'd had an affair with previously while I was pregnant with DS. They were together for about a year.
DS told me much later that he was terrified his Dad would "make her my Stepmum" because she shouted all the time and frightened him. Ex-DP used to take DS to her place (without my knowlege) and they would drink themselves stupid and have screaming arguments. DS would have to stay there overnight and share a bed with her DD !! because his Dad was too pissed to organise a taxi for the 1 mile trip home.
After they split up, a friend who works for Social Services put her job on the line and told me that this woman had had her DD taken into care for 3 weeks after she was arrested for being piss-drunk in a public park while her DD was with her.
I will never forgive myself for allowing DS to be exposed to all this, but I truly had no idea it was going on.
I do hope you can sort this out.
HTH X