Sorloc,
Good for you!
I'm glad that you let the kid dress up the way he wanted to, rather than force him to do something he didn't want to in order to please the grandparents.
The whole point of playing dressup is to be what you want to be, not what other want you to be (he will probably get enough of that if life without ruining Halloween). If he wants to be a zombie, he should be a zombie. He7ll only get a handful of chances to do stuff like that.
Way to go!!!