HELP!!!!!!!I HAVE BOY DRAMA

Ok so there is this guy,he considers me his best friend recently he Had been flirting with me (calling me honey,baby ect.) and saying that we're in a relationship .I really don't like him that way plus he has a girlfriend that is my friend also,they are still dating they just don't talk much so he has become closer to me I'm trying to break it to him that I can't be his girlfriend I just don't know how bc he is always depressed and says he needs me in his life I would like to break it to him but idk what I should do HELP PLZ