I am not a lawyer either.
To Gillette: from the OP's description, there was no sex happening at the party, so unless provocative dancing is considered sex, then the OP has to prove the bar owner is having sex with the 17 year old and that's off topic.
I don't know what to think about your course of action. I'm trying to put myself in you and your boyfriends shoes but I do not understand your line of thinking on the subject. You're clearly reacting emotionally and are not logically thinking this through.
If it was a private party, then clearly Liquor Laws don't apply, though supplying alcohol to a minor was an infraction and you could try to proceed on this point. But do you really want to?
What do you gain out of blowing the whistle on this? What are you truly interested in doing here? Your grounds for being "upset" about the situation seem shaky at best. Are you just trying to protect this innocent little 17 year old from exploitation? ....Because clearly, this person is not that innocent, and he's made some decisions and a judgement call, all on his own, that has put him where he is. Let him take responsibility for his decision; don't try to use law to justify your feelings. You sound like an upset parent. Believe me, all the kids I knew growing up were sexually active at 17, and were drinking, smoking, doing drugs, etc. I didn't see their parents going ballistic (and I'm sure they knew). And the end result was those kids have turned out just as well-adjusted and fucked up as ever other person on earth. Do you really want to take on this responsibility?
If it seems like I have strong feelings about what you're doing, I certainly do. A friend of mine who I've known since I was 7 years old, and is a stand-up guy, was an active, involved community leader, and a medical hero, went to jail for two years for having sex with an underage girl, who (after he ended it with her) became spiteful and vengeful and took advantage of California law (which is absolutely heinous) to try and ruin his life and to a large extent, did. He made a bad call by being involved with her - I make no excuse for that, but this girl had significant problems too.
If this 17 year old was in shackles and chains, crying to be released, tortured, or in agony, OR if it was at all apparent the kid was present at the party against his will, then, beyond reason or doubt, you'd have every incentive to be upset and take action. But, that's not the case here, not even close.
If you are trying to 'save the guy' and be the voice of moral judgement and ethical values, you're way too late and no 'enforcement of the rules' on the people or establishment who led this kid to being at the party and dancing around is going to prove otherwise. It's just going to cause a major headache and heartache (most likely) for the bar owner and all whom they employ, but also as Bigjpgh said, a lot for the kid as well. Again, do you really want to?
I also see absolutely zero rationale for why you posted this on Facebook...were you looking for support and people to say 'go for it"? This situation could blow up a lot bigger and a lot more quickly than perhaps you're prepared for.
I don't think this is worth it for you. But that's IMHOP.