Clearly you'll all make your own decisions. OTOH, I'm actually a certified behavioral specialist, and that's the POV I'm coming from, so I hope it's okay if I clarify further...
This isn't your average teen - my advice would be different for that sort of kid - this is a kid who is adjusting to life on the outs, who is at a tenous place right now and needs (IMO) time to settle in. He probably already feels rejected, and feels badly about himself, and is feeling unsure about whether he can handle life outside, and whether you'll truly accept him, and the easiest way to handle that is to just make yourself as difficult to be around as possible so maybe you'll get sent away again, and then you won't have to wonder and worry...
I had kids come out of residential into my class, and they'd do this. In the beginning, I was careful to keep my expectations clear and minimal. Hurting yourself or others was not permitted. Damaging property was not permitted. The rest generally wouldn't get you any positive strokes, but you wouldn't get any negative attention for it, either - and a lot of these kids were stuck working for negative attention because they'd given up on getting positive attention.
Eventually the swering needs to stop, yes. And if you truly can't ignore it, then it probably does need to go on the "not permitted" list. It truly didn't bother me coming from these kids, maybe because I could understand the reasoning behind it.
FWIW, in a fairly short time my kids all stopped swearing, except under extreme stress, and eve then they didn't sweat at people anymore - and that's how and when I swear, anyway.
Single mom to Rain (1/93) , grad student, and world traveler