I'd say that from reading what's posted here, and basing everything I'm saying purely on that alone, they didn't just "jump" your boyfriend's cousin, he probably said something or did something to aggravate them (yes, they may have said something to him, but he probably mouthed off to them).
You're "getting someone to sort them out".
Violence begets violence. It isn't the way to solve problems. It's a very immature and stupid comment to say in my opinion.
Let the correct authorities sort the problem out. Whoop-dee-doo. Your car got a kick. Thankfully it wasn't your head. Count your lucky stars that you or your boyfriend weren't king hit, fell and cracked your head open and died.
Let the police do their job. If they don't come in a reasonable amount of time, call them again, then again, then again.
This is now the second time an incident has occured, so this gives the police more of a reason to ensure that they're locked up longer next time. The police can only go by what the law says, and that is why they were free on bail the next day.
With an attitude like that above, there is no sympathy from me.