If he's anything like I was when I was 12, he thinks of himself as grown-up and able to make his own decisions. I'd agree with Lawren's advice to tell him all the positives and negatives of going dressed at Disney, and then let him decide.
As far as the police/Child Services getting involved, again, at 12 he probably feels old enough to decide things for himself. If there was an issue, all it would take would be him saying that he chose to dress that way, and they'd probably drop the matter. Crossdressing by choice is not considered child abuse, as far as I know.
Does he even want to go dressed to Disney? I've seen you ask if he should be allowed, but you never said if he asked you if he could, or if you just thought it might be a good opportunity.
Definitely sit down and have a talk with him. Find out how far he wants to go, whether he just wants to crossdress or if he actually feels like a girl.