It sounds like your son has mainly recovered from the issues from his past relationship and has moved on to a situation where he's happy. That's great. I understand that your son (and possibly you) may feel betrayed that family members are still friendly with the ex given that they know the impact the relationship had on your son, but perhaps they believe the ex has changed, too?
If the contact with the ex is solely to see your grandson, can your son actively arrange to spend time with these other family members during the time he has custody of his son? It sounds like it's more than that, though, for the person who said that your gs and his mother "come as a pair". There's just no way to dictate that your mother, sister, etc. cut the ex out, although it's reasonable to ask them not to invite you or your son to events where she will be. This should all be discussed in person, if possible, or at least by 'phone or email - not by reading each other's social media.
... my mum has cut my son off cos he dare to question my sisters totally screwed up thinking. Nothing you've said here indicates that your sister's thinking is "screwed up", although I appreciate it may be one of the details you said you'd not included. I think it's understandable that your mother may not want to hear her grandson speak this way about her daughter. Hard to say, but it seems like your son and his grandmother should be able to work this out between them without causing havoc in the family.
I understand your son may legitimately think that his ex is putting their son in danger by having strangers in the house. Unfortunately, this is a common problem when couples break up and need to raise shared children. Unless there's evidence that would stand up in court re the danger to the child, each parent can make their own decisions during the time they have custody. The babysitting part doesn't make sense - if your grandson is in danger when there are strange men in the house, wouldn't he be safer away being looked after by his grandma, aunt, or other family members? Saying no to that just means he'd be in the house with the dangerous men. Having family go through your son when the ex asks them to babysit sounds really inappropriately controlling - after all, if she asked a friend to babysit instead, your son might never know about it.
Do your son and the ex have 50/50 custody of your grandson? If so, the best he may be able to do is give his son the best care while he has him, and keep family members involved during his custody times (which he can't do if they're not speaking). IF he feels there's a case for him to sue for full custody, that's something to seriously consider - but realistically if he has had recent mental health problems that may work against him there.