Firstly, how great is this guy? Because work flings can be risky and can make things incredibly awkward! But if he is totally worth it, I would say you need to take a risk. Maybe casually ask him what he is doing on the weekend, and if he is free suggest a movie, or a coffee! Sounds scary right, well I guess you have nothing to lose right? Another option is seeking him out on facebook and striking up a conversation. Hope I helped!