I know a kid who didn’t get into his top choice. He spent his whole freshman year at another school pining for his first choice and hoping to transfer, but didn’t get the grades. To be honest, if your son didn’t get the grades in HS to get in, it isn’t very likely in college that he will have what he needs to transfer, either. I think the kid I know who spent freshman year focused on another school was sorry later that he had done so. He treated his friendships, classes, etc. as transient. It really took him until junior year to accept the school he was attending. I think he now wishes he’d just put the rejection aside and focused on getting the most out of the school he attended.
But you can’t make your kid see that. It is too late now to try to pop the ‘dream school’ bubble. I guess I’d humor him for a day or two with ice cream and distractions. After that, I’d probably change the subject if he kvetches. Help him plan visits to accepted student days at some of the options he has – those will help get him focused on the choices in front of him.
Oh, and the “I’m going to let my grades tank” comment? I wouldn’t take that from my kid for a second, not even in the first day of disappointment. I’d tell 'em that if they do that, all bets are off as far as financial help from you for college. If they can’t hold up their end, you have no reason to hold up yours. If he kept on that path, I’d let him know that spoiled brat behavior isn’t an option – it is a first world problem to not get into UCLA, and he better figure that out in a hurry.