How can I stop fighting with my friend?

I'm a girl and he's a guy. We go to the same high school together. We haven't met for very long, barely 5 months but we're good friends. I'm best friends with his girlfriend and he's my boyfriend's bro or buddy (whatever guys call it) so we help each other out a lot. We always talk over chat and we're pretty close. He trusts me with personal things and I trust him. Our problem is that we keep fighting again and again over small things. We could be happily talking about something and then the atmosphere would completely change to hostile. For example, he knows that I hate profanity but he does it to make me mad on purpose. I just want to stop fighting because it's really stupid and childish but we both get so angry. Help?