Alpha maleness isn't solely defined by how aggressively dominant someone is. To be honest, a lot of the time that kind of behaviour is a sign of insecurity. There are a couple of ways you could approach this:
1. you should work on your own alpha qualities. Work on your voice projection, for example, that way you can speak over him and always be heard better as the central guy of the group. If he tries to interrupt you with some childish comment, don't even let him speak. Just talk over him and if he tries to raise his voice to a volume that seems unnatural in relation to the way he's been speaking all along he'll just seem try hard. Use lots of touching too, ever notice how the alpha guy in any group can seem dominant of the other guys by touching them. For example, when you're talking to him have your hand on his shoulder in a dominant kind of way. Almost like you would with a bratty little brother who you just met out in a social situation and you're giving him advice on how to approach a girl. When he approaches you could even introduce him as "my boy...". You get the idea!
2. Reframe the situation, as someone mentioned already. Make it look like he's trying to impress [u]you[/u]. Like you are the alpha and he's vying for your attention. So for example, if he comes shaking you from behind you can give a slight smile and say something like "I'll be with you in 2 minutes man, just talking to these guys. I think [insert random name here] is around here somewhere if you're looking for someone". In this example your framing him his the guy who's on his own, who has no social skills and has no one to talk to. You on the other hand are a cool, social guy who's enjoying a conversation. Despite the fact that you're having a conversation, you're still nice enough to come and talk to him in 2 minutes if he's still on his own.
3. Another technique is to pigeonhole him. When he starts acting up you can say to the group something like "have you met this guy... he's like the loud guy of the group". Say it while laughing and putting your hand on his shoulder as mentioned earlier. Say it as if taking the piss out of a friend but as soon as you've said it put back on a straight face and say something like "no i'm just kidding, he's a good guy". This way you are giving him value... you're being a perfectly nice, social guy but you are the one who designates his value.
All of the above should be done with the correct body language. Don't face him... keep your body facing towards the group and just turn your head back to speak to him.
I have some more stuff on this... I'll post it later, just gotta rush off at the moment. Hope some of this helps though
