Short of gagging this child and bundling him away into a closet somewhere, what exactly is the school supposed to do? I might only be a training teacher, but already I've learned two things about discipline in schools.
Number one: if your lessons are engaging enough, the vast majority of your behavior management issues will be solved before they start. In other words, look to yourself first and ask yourself whether or not your lessons are engaging your children.
Number two: there are always going to be children for whom the normal behavior management programs just don't work. There are kids who are self motivated. There are kids who are motivated by the promise of the proverbial carrot, and kids who are motivated by the fear of the proverbial stick.
And then there are kids for whom none of these things are true. They're the ones who are on individual behavior management plans, and often, they're the ones who have things going on behind the scenes: sub-optimal home life, SEN, childhood trauma...
The important thing for you and your son to remember is that while the swearing is continuing to happen, 1. your son knows not to do it, so he shouldn't mimic; 2. the school is very unlikely to be doing NOTHING about it, they're just not consulting with your son before imposing this child's individual consequences on him/they're not placing consequences on this child in front of your son like some sort of twisted performance art piece; and 3. whatever is going on at home for this child, and whatever plan he/she is on at school, is none of your business.
So you could talk to the school. I can tell you what would happen. You'd be told that "they can't discuss that child with you", and that "they are dealing with it". You'd go home feeling like they blew you off (they did, and rightly so) and you'd be back here complaining about that, and the teachers/staff would have a new Joke of the Week to laugh over in the staffroom, neatly provided by yourself.
Far better idea? Educate your own child. I don't mean pull him out of school. I mean be a parent, teach him that not everyone is the same, everyone has different needs, and that he knows what the rules are and it doesn't matter what anyone else is doing, he needs to make sure HE is making the right choices.
My mantra for tattlers/complainers? "You manage you, and let us manage everyone else. Thank you very much, off you go."