I'm scared I'll annoy him by messaging first? Please read?

I met this guy last month (June 11th) at a party (his roommate was having it and invited me) and after hanging out with him a few times (like 2-3) I started liking him more and more. So I added him on facebook and messaged him one night and we started talking through that just casually and the other day he messaged me randomly and said "do you want to do something sometime" so I was like yeah sure :) and he was like cool and we didn't talk much after, I asked him whats up and all that and yeah. We've never just hung out before - i've only seen him when his roommate invites me over and only one of those times it wasn't a big party. Now should I wait for him to initiate it or should I ask him? Like ask him what he's doing on the weekend or something? How do I ask? Should I wait for him to initiate it because he's the one that asked to do something in the first place or should I ask? I really don't want to seem pushy but I'm worried he's already losing interest