There is a similar thread to this on the US Trip Planning board at the moment (has had a few strongly worded responses

). Apparently, it has been known for the CMs to ask the child how old they are so, unless you want to tell your son to lie, I would just bite the bullet and fork out the extra for an adult pass.
As an aside, personally I think it's daft that kids are considered adults at the age of 10 onwards but I am assuming it is all to do with the height restrictions and the fact that, at 10, the majority of kids can do all the rides. Even though I don't agree with them, I wouldn't dream of trying to bend/break the rules. In my eyes, that's deception - and I certainly would not encourage my children to be untruthful.