Ok so I happened to look at my son's texts (never done this before and certainly wouldn't want to do again). He's just 14 and some of the language (not just him but friends, girls and boys equally), they are using could make your hair curl. I don't want to come across as an old fuddy-duddy, but I feel so let down by him, and upset that he uses this language, but mainly because it involves sexual stuff. Like other people have commented, the stuff on Facebook and texting, parents find increasingly hard to "police". What would other parents do? I don't want to be confrontational, as I really don't think that would help matters.
Don't tell him you peeped or he will pin protect his phone . Maybe just have a conversation with him reinforcing the fact that while he may hear and see foul language he does not have to use it. Remind him that anything he types into the phone or computer could be stored forever and he may regret it when he is older.
I had a discussion about the same thing the other day with my DCs after seeing some things on Facebook. I decided that as long as they speak properly infront of adults, and know correct grammar/spellings for schoolwork, did it really matter in the overall scheme of things ? I remember being a teen myself (all those years ago ) and the things I used to talk about to my friends. All that happens these days is that they get written down in text format so look much worse - it is a natural part of growing up to talk about sex, relationships etc with friends and they do tend to have friends of both sexes these days. Also remember that quite a lot of teen talk is bravado and not necessarily in the realms of the truth. As long as the language/explicit stuff isn't creeping into everyday life, and is limited to discussions between friends and doesn't affect attitude/behaviour, then I would possible pretend that I hadn't seen the texts. You could also try to make it into a light hearted discussion along the lines of "when I was young we used to do/say xxx , or have a slang word xx for xx" to see whether he opens up and talks to you. Of course, this does depend on the explictness of what you saw and you may feel that you have confront him about it.