What do I do if a guy asks me out while he's going out with another girl?

Okay, I'm 12 and i'm in 7th grade, this guy, JB (Jonathon) asks me out, but he's going out with another girl. What do I say? He's an 8th grader and I sort of like him, I texted him Hi and I was trying to ask him out but not literally, then I asked "Can I ask you something" He said "Ya" I said "If you had a chance, would you go out with me? I'm just wondering." he then said "Ya"I said "Oh." He said "Ya" I asked: "Who do you like?" He said "Idk"I then asked "Are you going out with anyone?" He said "Ya" I replied: "Oh..." He said "Ya" then I said ">.<" and he replied: "What?" I said: "It's a face" He said: "Oh" Then I said " :/ " Then he asked me: "Y do u want 2 go out?" and I said "Well" then hee asked: "So do u?" and I said "Yes" then he said "Oh dats kool" I'm afraid my bff is going to get mad, and my other friends, too. And he asked me out while going out with someone. WHAT DO I DO?! Help!!