Question about my co-workers behaviour?

I think he has realised what he missed out on when you first ask him...I'd definitely approach him about it and just mention that he has been acting a bit more flirty and would he maybe like to go out on a date with you now. It can't hurt, he seems like a decent guy. Just be careful that he doesn't use you though - as he said, he's a bit 'messed up' and if he knows that you're into him he might take advantage of you. Good luck!!