Hi Paul,
I tend to agree with Marc's advice. Have a talk. Bad language reduces one's credibility. Teenagers need to be accepted into their group and behave accordingly to achieve that. Even when that behaviour may cause others to lose respect.
This is what a teenager needs to be made aware of.
To go checking on the young one and he becomes aware of it can cause much damage to your relationship. Strive to trust him rather than be "Big Brother". Of-course he will have to earn that trust.
I used to tell my kids that I trust them untill they did something wrong. They may be able to do things and get away with it for a while. Eventually they would be caught. I treated them with the respect that adults have. I treated them like adults. They never abused that privilege. Or maybe I never found out.
Treat them like kids, they behave like kids.
Cheers