What to do about this guy at work? (I already have a bf)?

It seems nothing more than innocent flirting so it shouldn't be a problem and as for what to do now just be a little more professional but that doesn't mean you can't still have a laugh. If at any point though you feel that things are getting a bit too much or you think he is taking your flirting too seriously then at that point it might be a good idea to let him know the deal and where he stands.