I'm surprised to refer to this on a non-programming SE site, but your question is a case of the XY problem. You're asking about telling him to not drink (Y), but in reality you're trying to get him to not be a jerk (X).
Ask yourself this: would you prefer him being a sober jerk, or a friendly drunk guy?
Playing the devil's advocate here, I think asking to not drink at a music festival is a bit silly. Plenty of people drink quite heavily during festivals. While that may not fit well with your group of friends, it still feels jarring to try and make rules about others' usage of alcohol. It's much more relevant to address their behavior regardless of whether they're inebriated or not.
Now of course you can't just single him out and blatantly ask him to not be a jerk, but it's relevant to realize that the drinking is tangential to the actual problem; which is rooted in his behavior.
Although you haven't really described what type of jerk he is, for the rest of the answer I'm going to assume that he's not malevolent, but rather socially oblivious, indifferent, or you guys simply don't match on a social level. If he is malevolent, you should simply steer clear of him as nothing you say will change his intentional behavior.
Asking him directly.
This is the harder option, but the most effective when done right.
From your question, I infer that he's the only person who behaves like a jerk. Which puts you between a rock and a hard place:
- If you phrase your request in general to everyone, it comes across as restrictive and, unless your friends are aware of what you're doing, can cause your friends to infer you don't like their behavior.
- If you phrase your request targeted at him, he will feel targeted (because he is) and it will antagonize him. This can increase the likelihood of him being a jerk, or it can even blow back on your friend (his girlfriend).
If you decide to approach him, make sure to not antagonize him:
- Talk to him in private.
- Don't assume he is going to be a jerk. Don't even use the word "jerk" (or any synonym).
- Don't tell him what he must (not) do. Phrase it as a worry, rather than a rule.
- You can turn it around a bit, and imply that your friends are sensitive to energetic behavior (as opposed to implying he is the problem) or apprehensive of people they don't know. Even if that is not the objective case, he will see it that way, so you're explaining it to him in a way that makes sense to him. Don't throw your friends under the bus, the goal is to avoid making him feel like he's the problem (assuming he is not malevolent or consciously dismissive).
- Never imply that he is incorrigible. Address it as a minor problem that can be easily fixed and forgotten about.
Something like this:
Hey, I wanted to talk to you about the festival. I'll be meeting up with a group of my friends. You guys are welcome to mingle with us. But I wanted to give you a heads up: my friends can come across as apprehensive or judgmental when they meet new people. Don't worry, they're great guys when you get to know them.
Just try to not come on too strong, (your friend, his girlfriend) and I don't want you guys to get off on the wrong foot, we'll all be there trying to have a great time.
It doesn't matter that you implied that your friends are unusually sensitive (rather than him being annoying). What matters is that you pointed out that he must approach this social occasion with caution, for everyone's mutual benefit (stressing yourself and your friend (his girlfriend) a bit more).
If you focus on how he's the problem, he'll never see eye to eye with you. But if you focus on incompatibility without blaming him, he may be more open to behaving cautiously to not cause an issue.
Asking her instead.
This seems like a safer approach, though it's more prone to failure. At least, I assume that is the case, because otherwise your friend would've already successfully addressed this with him (or is she oblivious to his difficult nature?)
The idea is pretty much the same as above, but you have a bit more leeway in calling him out as the problem, because you're not talking to him directly. However, this heavily depends on whether she understands that he can be a jerk. If she's aware of it, you can talk quite openly (but still avoid taking it too far).
If she is oblivious to his behavior, then you have no extra leeway compared to above. In that case, you risk antagonizing her by openly criticizing her partner.
- Talk to her in private. Preferably not when he's nearby, even if out of earshot. If he's nowhere nearby, she may be more open to talking honestly about him.
- Don't assume he is going to be a jerk. Don't even use the word "jerk" (or any synonym).
- Don't tell her what he must (not) do. Don't assume that she'll be his "social caretaker".
- Focus on mutual incompatibility between him and your friends, avoid playing the blame game.
- If she does not know your friends yet, you can similarly turn it around a bit, and imply that your friends are sensitive to energetic behavior (as opposed to implying he is the problem) or apprehensive of people they don't know.
- Never imply that he is incorrigible. Address it as a minor problem that can be easily fixed and forgotten about.
Something like this:
Hey, I'm looking forward to seeing you at the festival. I'll be with my friends, but you're welcome to hang out with us if you want. I am slightly worried about [boyfriend] though. My friends are generally apprehensive of new people they meet, and I worry that they might get off on the wrong foot if he comes on too strong.
I would expect her to reassure you at this point. If she doesn't, and isn't oblivious to her boyfriend's difficult nature:
I just hope it won't be awkward. Do you think it will be? Should I talk to him about it?
Again, I'm expecting her response to be reassuring. And if she agrees that you should talk to him, you might be better off actually doing so (it implies that she won't be able/willing to communicate that to him).