I'd suggest a slightly contrary view. Give him the story as it's told in the bible as best you can. God, the creation, Adam and Eve, the Nativity, the life of Jesus (chuck in a couple of miracles), the Crucifiction, and a brief description of what a church is. That should prompt about a zillion questions. You can reasonably say that some people believe in God and some people don't.
Most kids go through a religious phase. Run with it. Help him make the most of it.
I'm with Rich...my kid is all mixed up now!
Tom Cruise is worth a mention now - Wow! (Courtesy of Gawker, as it has been removed from the tube)
Hold onto your seats!
We explained to ours that there is no right or wrong religion, and treated religion as a series of ideas that people had and told them about the main faiths. We are aetheists [if we have to have a label!] and have left our children to decide for themselves as they got older [now 22 and 19!]
We've explained to them over the years that the main idea to draw from religions is the personal moral code that everyone has and how that should form the basis of how you treat people- that they should react to, and respond to people in the way that they would like to be treated by others and that that should form the way they dealt with any situation: to be aware of their personal responsibility and let their conscience mould how they respond.
All our parents would class themselves as Church of England but apart from school Christmas plays our children have had no organised religious input.
I trust you explain that a person can have a good moral code without religion and a poor moral code despite having religion in their lives.
I think I'd go with the 'some people believe XYZ and they are called Christians, and there are others who believe ABC and they are called Muslims' etc. I reckon letting kids make up their own minds is definitely the best way to go.