Springchicken I'm going to put aside the religion in schools argument for a second, and go back to the individual case. I'm going be honest with you here, and I hope you take it in the spirit with which it's intended...
You sound just really desperate to control this situation, and I genuinely don't know why you're so worried? Your DS is 6, FGS, he's not going to make up his mind about his spirituality for a VERY long time, and any paths he follows now are highly unlikely to last.
So what if he came home tomorrow and told you he was going to be a Buddhist? It'll last all of 5 mins. And as he gets older he will be perfectly entitled to choose whatever faith or lack thereof he wants - that's what being an adult is about.
You are more defensive about your belief system than any religious person I've come across in a long time, but really what you should be doing is informing your DS what you believe, explaining why you DON'T believe in any religion, and also telling him ALL ABOUT the other beliefs in the world.
Taking this path of uber-censoring is going to end in one of two ways: (a) as your DS gets older, he will know exactly what buttons to push if he wants to rebel, and you'll find yourself with a Jehovah's Witness for a son, or (b) you will end up with a child with a patronising and unpleasant zero tolerance of anyone whose beliefs differ from his own.
Are either of those scenarios acceptable to you?