I'm struggling with that, too. I just had the first conversation with my parents about my atheism. I have a 16 year old daughter who knows my views, and an 11 year old who still believes in God and Jesus. Telling the "truth" isn't as easy as it might appear. My main goal is to allow my children to discover the truth for themselves. They may come up with a different truth, and that's totally ok with me. The one thing I DON'T want to do is proselytize to them the way my parents and elders did when I was growing up, even though I am confident in my views.
What I do with my 11 year old is basically avoid specific questions and tell her that she has to figure out what she believes on her own. I will certainly help her research, but the only influence I want to have with her regarding this subject is to tell her not to believe just because someone says it's so. Find the answers by doing lots of research. If something doesn't make sense, you should look at it with skepticism and examine it until you can either accept it or dismiss it based on fact.
Congratulations! You've raised a kid with an inquisitive mind and a love of knowledge!
If you want to remain as Neutral on this as possible, simply tell him that God is many things to many different people. Explain that many people all over the world have different ideas as to who or what God is, and explain that some people don't even believe in God at all. You don't have to lie, and you don't have to tell him you're an atheist if you really don't want to.
That said, it might not be a bad thing for you to tell him how you yourself feel about God. It would be a prime example for him of being who you are in spite of what other people want you too be. You can hide your religious beliefs from him if you think it will protect him from the ugly side of the Theists, but you could also take this chance to show him what strength of character is, what true virtue is, and what morality is.
You really can't go wrong here, as long as you don't straight up lie and tell him you believe in a god. The old saying "The Truth will set you free" is kinda perfect for this situation. If you do decide to go with the full disclosure route, you need to be ready to put out whatever fires it might cause. I'm not saying it will be easy, I'm just saying it might be the best in the long run.