Just be like “Hey, I’m really not comfortable doing this. Thanks, but no thanks.”
The problem is, he’s legitimately a nice guy (note: not a Nice Guy, like an actually nice person), so I don’t want to say we can’t hang out at all. I just really don’t want to be his girlfriend.
I’m with blue. Just tell him that while you enjoy his company as a friend, you feel as though he wants more than platonic companionship and that you aren’t comfortable with having any sort of romantic relationship with him.
This isn’t gonna be fun at all.