I don't want to be his girlfriend so how do I reply to this?

Just tell him that you are not ready for a relationship yet. He'll understand and he'll wait if he really likes/loves you. If he doesn't want to wait, or he gets mad, then he may not be the one for you. Don't rush into something you are not ready for. I was in this same sort of situation (except my bf isn't a rockstar). This guy that I liked asked me out. We were really good friends and I didn't want to make that friendship weird by dating. So I told him I needed time to think. His exact answer was "take all the time you need. I'll be here, waiting for your answer". One day, I answered him (yes, by the way). We are still together now. So, like I said: If he really likes you, he'll want to wait for you.