How do I proceed? Exwife has told Ds (15) to "go play with razors"

[PeanutButterSquash]

potentially triggering, mental illness

I have a 15y old ds, as well as 3 other dc's.
He is under CAHMS for self harm, depression and bulimia.
Exwife has made some horrific comments to DS about his MH issues, ds has come home from a weekend there and told me exwife has said (among other things) "I'm sick of the sight of you, go fucking play with razors", this is because she caught him attempting to throw up his meal.
I did call her and she skirted around the issue before finally confirming what she had said (the same as DS had claimed). I'm honestly at a loss now.
Ds doesn't want to see her, what do I tell my other kids? Ranging in age from 7-19. I have no idea what to do and DS is absolutely beside himself thinking his mum wants him to die.
My initial instinct is to keep my youngest 3 away from her for the time being while I access professional support and opinions about this as I feel this is very abusive. My eldest (19) can make her own decision, though she does live with me.
I'm so shocked

Oh your poor son I dont really know what advice to give but that's definitely abuse.

Surely at 15 he is able to decide?

At 15 he is old enough to not see her . Frankly she sounds unhinged , possible I’ll herself frankly . Right now I would focus on your DS . Reassure him he is loved and cared for . Explain people can say evil things to people they love

Poor lad . You sound like you have a lot on

Also write a full account of what happened . As if you don’t you might forget some bits

She doesn’t deserve him

Is supervised contact a possibility for the younger children.

Do all the children live with you on a permanent basis? How is your care shared?

I'd be tempted to take a week time out to assess the situation, speak to your ex, put plans in place or your son. How old are the youngest, would they accept that they are not seeing Mum this week because (insert your own reason!)

At 15, he can decide and it is probably best to seek more professional advice regarding contact with his younger siblings. Whilst what she said was utterly and completely unacceptable and really damaging to your son, I would not close the door completely to her if she gets help. You would be surprised at how many loving relatives of people with mental health issues can say things like that when they are just really terrified of losing their loved one and angry that their loved one is hurting themselves. I know it still doesn't make it right and, if she isn't in control of her own emotions right now, then she isn't really in the best place to be looking after children, particularly vulnerable ones. Your focus, however, has to be on your children. I hope your son is getting all the help he needs.

The kids all live with me full time, with various contact arrangements.
Ds only goes on alternating weekends - his choice. The younger ones go on alternating weekends with 1-2 school pickups. On these days Ex will take them out somewhere until 6pm or so then bring them home. The eldest obviously goes whenever she feels like, which varies as she is in college.
I don't think ex would take kindly to supervised contact tbh, I'm not sure how I go about arranging it I'm going to talk to DS about this and previous comments (much more tame but still not ok in my eyes) and compile a log. I don't know how to go from there, though: social services, maybe?
This is a huge shock to me and I can't believe she has done this

If I were you, I’d check with the younger children to ensure if she’s abusive to them

It may be she’s fine with the younger ones, she she just can’t cope with your son. If that’s the case he’s old enough to reduce contact with his mother and the younger ones can continue to see her.