In answer to your question - I am terribly concerned far too much of the time!!
Have a DS who is 6 year's old, quite young for age, not perfect but normal nice little boy with a good heart. Friend is a neighbour who is 7 year's old but as you so well described - so much older in his head. Not malicious, but very angry with a very negative (wear you down) attitude, quite rude and no manners. (Two examples: 1. Using third finger at bus driver after being dropped off after school. 2. Will intentionally ignore in rude manner adults inc. DH when saying hello/goodbye etc). I would also describe him as being quite rough.
The way I deal with it, is quite sternly, in that I now do not allow ANY rudeness ie. bad language, bad manners, bad attitude in my house. Because he absolutely loves coming here - lots of space outside, toys, freedom to play as much as they like, nice snacks etc etc., it actually does work (hope I'm not tempting fate there). But half measures don't work with him, and it makes me feel quite old and strict sometimes, but I simply cannot allow him to dictate the mood/bad attitude under my roof.
Like you, have started a couple of after school activities for my DS, and I am sorry to say, but although I would take my DS along anyway, I am very conscious of the fact that it's spending less time with this not so great character.