If you purposely set out to teach him that JW's and or religion in general is bad news or that the Bible is nothing but nonsense, you are just as bad as they are.
If, when he asks about things having to do with holidays, customs and religious beliefs that he's heard of, you show him what religion is and help him an overall general understanding of the different beliefs out there and how to look at those beliefs critically yet without judgement of others, he'll be able to see things for himself and realize that its ok to do so.
Having grown up as a JW, I was told what to think and believe about every possible topic before I even wondered about them myself. Not only that, I was told that I would be a huge disappointment to everyone I loved and I would even die if I didn't believe what I was being taught.
I think that it's unlikely that any child who is free to look into whatever interests them and is taught that it's ok to come to his or her own conclusions about things without the fear of being judged or a disappointment to his family, would be readily taken in or mislead by a high control religious group .
I think you should relax and enjoy your grandchild and trust that when the time comes, you'll be able to show him how to see things clearly.