Evening Starry, hope you are well on this winter's nite.
You are in a good position in that you've seen him out socially. You have a subject about which you can start a conversation. Maybe make a comment about the club, how crap the dj was, did he have a good time etc.
If you didn't see him with anyone then ask him if his girlfriend had a good time as well. If he says ' Haven't got one ' then you could arrange to meet him somewhere the next time you are both out before you end up at the club. Don't meet at the club as conversations are hideously difficult to hold in a nightclub.
If he says 'She enjoyed it' or 'No but my boyfriend did' then I'm afraid you're out of luck.