Well you already suggested it without words. At this point you'd look kind of needy if you followup again saying "Let's hangout out". Maybe he's just nice and not interested or maybe he's just a pussy and can't ask you out.
Perhaps on his own end, He is a bit of this Coy Boy and may Feel... I'm still Not certain if She likes me. Go head and Make the first Move. I bet you will Tweet like a Bird if he would Say "Sure," And go Beyond the 'We've been messaging each other on twitter.' Good luck. xx