You should simply say to your son that when people and animals die, they do not come back. And if someone says that a dead person came back to life then it is either a made up story, or some kind of trick and the person was not really dead in the first place.
Also say to him that life is really, really precious, and we should not trivialise it by polluting our minds with imaginary resurrections. Why would god be so greedy and only allow his own son (ie himself) to come back to life? Why not bring back all dead beings? After all many of them died unfairly.
I think it would be best to share both points of views in a simplistic manner (christian/atheist) and then tell him that reserving his choice on which side to take is not a bad idea. Telling a kid what he shouldn't think would just spawn rebellion so I can see how this can be a troublesome issue. If I were you, I'd focus on getting him to understand the importance of an open mind, taking in points and views from all aspects and analyzing them at his own leisure. This will relieve the pressure of trying to make such an early decision on a life-changing belief system and will let him focus on other things (like school/having fun).
I think explaining the world of science here and there will have him automatically start questioning religion. From there you just have to let him know that it is OK to question religion (and everything else this world offers for that matter) because the smartest people are the ones who ask questions. By asking questions, I'm sure he'll come to the many faults of religion.
Those are just my thoughts right off the bat. If this doesn't help then I hope you find an adequate solution to your liking soon. The early years of a child's life are extremely important. I would go as far to say that they literally shape their thinking processes and what they define to be "true".
In any case, good luck!!
LSM & Peter, both of you make good points, but I think that approach wouldn't suit a 5 1/2 year old kid.
At that age they aren't mature enough of grasp the concepts religion, atheism and science. At that age, everything is like a fairy tale. Its healthy to let their imagination run wild & maybe dabble in a little religion, specially during festivals. With proper parenting they should come out of it just fine, just like kids finally stop believing in Santa Claus & the tooth fairy.
Eh? Where did I mention religion, atheism, or science?
The fact is the poster WANTS some advice in relation to what to say to her son. The kind of fluff you mention is not really any advice at all. You want the kid to accept that all fairy stories are equal, well I bet he understands that they are not if someone bothers to tell him.
He's not too young to learn that a galactic zombie coming back from the dead to roam the earth and then fly back into space is probably just a made up story. And here, look, there are many like it, so they can't all be true.
This is an extremely vulnerable age, and I think good parenting is knowing which fairy stories to let your kids dabble in. Santa is not a problem, as he'll grow out of it - because it's not true. Jesus is passed off as true by adults, that's the difference. The Santa story is harmless fun that we all participate in as nothing but fun. The Jesus story leads to violence, social exclusion, and bigotry. A kid should be shielded from this rampant evil nonsense.
The religion & atheism part was by Peter.
And I think the advice I meant to give was not to be too forceful about things. Its one thing to tell a 5 y.o kid that what his friends are telling him about this Jesus fellow isn't true but what if he doesn't wanna believe you but would rather believe his friends who tell him all the 'wonderful' things about religion. What do you do then? The kid is only 5 & won't turn into a devout Christian overnight. Yes, it is a very important issue and needs to be handled carefully but not immediately. The boy should be able to think for himself and not be just told that god doesn't exist. Wouldn't that be similar to pressing religion onto him?