To encourage my son to break a friendship and contact friends parents as to reasons why.

That's a wonderful update

Agree with first sentence . I’d really encourage his confidence by giving loads of emotional support - this makes a huge difference to a child’s decisions

Totally agree with all this

If this other child was mine I'd like to know this - it's this child that has the real problems. Sounds psychopathic. But people are never willing to hear their child is a bully from another parent, and making contact yourself won't help and may exacerbate the situation.
Good advice here about how to help your son to be resilient in the face of this bullying - blocking, other social outlets. Keep calm and steady with your son.
Tell school and the Police because this child may escalate their claims about your son and put him in jeopardy. They need to be stopped.

At 15 I wouldn't call the other parents. I would encourage your son to cut him off and meet other people.

Do not contact the mother. It would almost certainly make things very much worse.

I'd contact the school, not just to report this behaviour, but to express concern about your son. If this is his only friend, and this friend is manipulative and cruel, as a parent I'd be worried about my son's choices.
He clearly needs support with his confidence and his social skills. So there are two parts to this:
a) that the friends behaviour is picked up on my the school, and b) your son needs support so that he is less vulnerable to being bullied or taken advantage of as he approaches adulthood.

@Pottedpalm not the schools problem when bullying is going on ?! You are wrong here. I'd hate to be your child going through this!

Stay out of it, don't call the mum, she won't do anything. Give your son good advice and let him follow it.

This definitely has the potential to transpire into something rather hateful if your son decided on the no contact approach and totally ignored him. I've done this before and ended up with police at my door after the offender graffitied some rather unpleasant words on my driveway and started to turn my other "friends " against me.

I wouldn't speak to the parents with both boys present but I'd ensure I have some evidence with me so when the other boy denies it, you can prove it.

This friendship can't be fixed and it does sound like a case of the green eyed monster for whatever reason. I'd politely tell his parents they need to deal with their son's behaviour before it escalates and the boys should stay away from each other and not involve or interfere themselves with other friends in their lives.

It doesn't matter how old they are, they are still children at 15 but definitely old enough to reap the repercussions of their actions

Is there a reason why your son only has one friend? Generally at 15 you would expect him to have a couple of good friends and then a wider bunch of people to hang around with. Would he like more friends or is he very fixated on this one person? Obviously a good option would be for him to branch out and become friends with other people at school which would dilute the nonsense that this person is spouting. It's also a bit unusual for a 15 year-old to ask you to contact the other students parents, does he normally expect you to step in and sort out friendship dramas? What does his girlfriend think about what she is being told?

Don't ring the parents as this little shit will have thought one step ahead and will just deny deny .

I would give my DS LOADS and I mean LOADS of emotional support to he’s confident confidence not to accept any nonsense from this boy - I won’t call him a friend of any sort.

i wouldn’t contact the mum / just not appropriate at that age.

What your DS has been through has been horrendous - I sympathise as went through similar at that age

Agree with others. He needs to get away from this child and I wouldn’t speak to the parents

Do not contact the parent. It never goes well.

I would contact school as I am assuming both boys go to the same school and would state the facts and ask for guidance from them. It doesn't matter that it didn't happen on school grounds. Ds2 was bullied in a computer game by a "friend" of Ds1's. We reported it to school, the child was punished and Ds1 dropped the friend immediately and told him straight no one treats his brother that way.

The 15 year old needs to understand there are consequences to spreading lies about someone and the threat of violence is incredibly serious. School deal with this sort of thing all the time.

The safeguarding policy document on the school's website will detail how the school will keep your child safe. Please contact school.

Bullying is very much the school’s problem and they should be proactive about addressing it when told.

OP do not contact the other mum. It will just make the boy even worse when he finds out your DS told tales on him. Encourage your son to stay away from this boy who is clearly not his friend.

What stands out to me is that DS wants to keep this 'friend'. That feels a terrible idea and I think your effort could be better used by supporting him to navigate ending this friendship and working on his self worth that he doesn't accept people into his life that do this. It's not a friend at all.

Perhaps he would benefit from thinking through how to open up his circle a bit more.

I'm not sure there's much to be gained from speaking to the other mum, especially if you don't know them already and given that the friend has already proven himself to be really unpleasant might make things worse for your son .

A student kills himself after being "cancelled " over rumours of sexual impropriety https://www.dailymail.co.uk/news/article-14060637/Oxford-University-student-20-killed-cancelled-female-friend-told-pals-felt-uncomfortable-sexual-encounter.html

100% this! ⬆️

Has this all happened since dc started having a gf?

Zero Suicide Alliance has a lot resources for all age groups https://zerosuicidealliance.com/

Suicide is the leading cause of death in this age group. This boy is begging for help

Do not get involved like this. Adults getting involved in kids' relationships never turns out well. At 15 it's time they were managing their own friendships. Offer advice, but don't get physically involved.

it has also transpired same “friend” when they were not friends set him up to be attacked by a number of kids This is very serious. What happened with the attack? I would report this to the school if they were involved in sorting out the initial attack. Was there any police involvement?