I am Autistic, so I may have a different perspective. It sounds like he has a serious sensory issue with people touching his head? Is it just when he is measured, or does he hate all forms of head touching (in the bath, etc?). Perhaps he only dislikes big objects like yardsticks over his head? See if you can pinpoint down exactly what sensation he cannot stand. Sensory issues are very real and need to be respected, but sometimes a person can choose to put up with uncomfortable stimuli in order to get something they want, like a ride. Sometimes no matter how much a person wants something, the sensory pain is too unbearable to cope with.
I would suggest what others have in terms of telling him in words he can understand, that in order to ride rides, the people need to measure him. See if you can find pictures online of the cast members with their measuring sticks. A social story might help. Disney also puts out a PDF guide to the parks which they say is for people with "cognitive disabilities", but it might be suitable for Autistic people without cognitive disabilities as well.
Make sure he is given the choice to either be measured, and ride the ride, or not be measured, and not ride the ride. His choice! Don't cajole him into doing something he doesn't want to so (be measured) but give him the choice.
Also if he chooses to be measured you could tell the cast member ahead of time that your son has sensory issues and hates to have his head touched and if they can do it as quickly or gently as possible that would help. Yo can even warn them that he might squirm or scream or whatever. You can disclose his diagnosis if you feel comfortable doing so, but please be mindful of what you say in front of him if you haven't told him yet! (And I do suggest you begin talking to him about being Autistic as soon as possible so that the first things he hears about autism are positive or neutral things from people he loves and not all the horrible things out out by the media and some mainstream autism organizations.
Third, your sentiment of hating the label of "autistic" is a very common one, but it comes from a place of fear and misinformation. Labels are just adjectives. You label this person your son and yourself "mom". I imagine you label yourself a woman, perhaps a wife. Perhaps you identify with a religion or ethnic group or are a fan of something. Maybe your son is funny or smart or kind. These are all labels! Autistic is just like that. It's only negative if you think of autism as a bad thing. Your kid is awesome, I'm sure! He's also Autistic! And there's nothing wrong with that.
In fact, a lot of autistic people strongly prefer identity-first language (I am Autistic) instead of person-first language (I have autism). I don't have autism any more than I have femaleness or kindness or smartness. I am autistic, and female and kind.
I know it can take a while to get to a place of acceptance and treasuring diversity, because there are so many bad things said about autism and disability in our culture, including by doctors. But your kid is a whole and amazing (and yes, Autistic!) person and I'm sure you already know that. I hope your trip is incredible and that your boy gets to do everything he wants. I personally find disney world to be a sensory-friendly place because they pay so much attention to atmosphere and sounds in the environment.
You may want to read this:
http://autisticadvocacy.org/identity-first-language/
And might enjoy a blog like this:
https://adiaryofamom.wordpress.com