I’m closeted and a guy has a crush on me. What do I do and say?

There’s a guy who has a crush on me. He’s very very sweet, but I will just never be attracted to him. What’s so ironic is he’s the roommate of my queer female friend, who I have a massive crush on.

It’s been kind of difficult to nicely shut him down because he hasn’t exactly made a move, but my friend has told me a few times that he has a crush on me. He asked for my number very casually recently, I gave it to him, he’s only texted a couple times, very casual, nothing romantic. So there’s been no opportunity to actually tell him “I’m not interested, but it’s not you…” etc.

I’ve never had to deal with this before because I’m usually invisible to guys since I don’t care to attract them (but I’m also too repressed to explore my sexuality).

What can I say or do? When (or if) he eventually does ask me out, what’s a nice way to politely decline? Any advice appreciated :)