Well wishes :) Grandparents are tough. Any mutual themes in what you believe and what they believe? So, you instead of cut them off, give them a narrow range of topics? (Although, I know with some people we give them an inch they take a mile...). I was raised in a small town and we had an "interfaith" council that set up summer bible school and the figured out what we could teach all the kids and still not offend any one religion -- although that was a group of Christians with simliar-ish believes. Any, again best wishes :)
I agree with all the comments about your approach allowing your kid to hear that religion is really important to his grandparents but also that people can believe lots of different things and that's okay. I think if you're worried about the grandparents proselytizing in a bad ways (i.e. x people are going to burn in hell) you can add that "if there's anything Grandma or Grandpa say that makes you feel upset or scared I want you to tell me about it so that I can help you feel less scared". I also think that for myself, as a person with strong values but no particular religion, that I try to be proactive in communicating those values to my kid to counteract some of the religious stuff out there - things like "in our family we love all kinds of different people" or "in our family we are kind to others". That way there isn't a "void" of values that other families might fill with religious beliefs, if that makes sense.
I do love some of the religious songs, though, because I love the music that comes out of Appalachia. Some of them I've rewritten so I can sing them to the little man, and some I haven't bothered. Like in "Balm in Gilead" I sing that "there is a balm in Gilead that heals the lonely soul" instead of "sin-sick soul", and that "the love around me" instead of "the love of Jesus" revives my soul again. But since I think of Jesus as a quasi-real guy who had killer progressive values and pretty good community organizing skills, I'm okay with leaving some of the religious stuff in there and having conversations about the more esoteric "god" and "heaven" stuff. Just my 2 cents, though. I'm sure you'll settle into something that feels right.
You know, I just re-read more carefully and realized that it sounds like you're asking more about what to say to the grandparents than your child. Sorry about that. Reading comprehension iz hard. ;)
I haven't had to have this convo, so I defer to more experienced folks. But it seems like it would be hard to go wrong by starting with "I know this is important to you so we're not going to ask you to not mention it at all, but please be respectful that we don't want him to know about things like hell or [insert whatever problematic ideology you think may come up]."
I've also only got well wishes.
Between Mr. S and myself, our family runs from atheist to nominally Catholic to very devout Catholic to various flavors and scariness of fundamentalist Free-will Baptist and Pentecostal to mega-church attending and really happy about Jesus (have you ever seen Jesus Camp--they're kind of like that) to vaguely spiritual but not really believing in a deity to Hindu. Religion has been off limits as a topic of conversation other than in the most general terms in this family for a really long time. It just makes life more pleasant for everybody that way.