HEHE...this always makes me laugh.
Sorry, YOU gave them his email address, they didn't simply guess it.
And I agree with Sarah...I don't think this is the type of gift you simply SURPRISE someone with. Especially since it seems as if your son is an adult. Chances are he has a family or at least a life...maybe he can't take time off, maybe he needs to save some cash, etc.
What you did was still very nice, and if this ruins everything for you, I would ask that you take a look around you and realize your life is pretty darn awesome.