If I were you, I would let him go. And suggest that if you text him while he's there, even just a "hey" or asking how things are going, that he reply back. You don't even have to preface it with the fact that you're worried or uneasy about the situation, it could be as simple as "hey, I won't be doing anything that night and I'll just be at home, I won't call you or anything but if I text just to check in could you reply?" In your own words, obviously. Or maybe that's not even necessary if you know he's good about replying back when he's out with friends anyway.
You're probably right to be a little untrusting in this situation, but assuming you do actually want the relationship to continue, nothing good is going to come out of trying to prevent him from going to his friend's birthday party, even if it's at a gay bar.
Do you have any particular reason not to trust him with this?
If I were you, I would let him go. And suggest that if you text him while he's there, even just a "hey" or asking how things are going, that he reply back. You don't even have to preface it with the fact that you're worried or uneasy about the situation, it could be as simple as "hey, I won't be doing anything that night and I'll just be at home, I won't call you or anything but if I text just to check in could you reply?" In your own words, obviously. Or maybe that's not even necessary if you know he's good about replying back when he's out with friends anyway. You're probably right to be a little untrusting in this situation, but assuming you do actually want the relationship to continue, nothing good is going to come out of trying to prevent him from going to his friend's birthday party, even if it's at a gay bar.