Be honest. IF you do identify with a religion and you want to start practicing, talk to your son about it and start attending services. If not, tell your son how you were raised, what your beliefs are now and why you don't go to services. Have him record those feelings. I guarantee, you are NOT the only family who doesn't adhere to a religion! Explain these things to him with authority. Are you an atheist or agnostic? If so, tell him why. It's perfectly okay to tell kids that you aren't certain about some things and you're not really sure what you believe if that's the case.
***Just a note -this isn't an inappropriate question from school. As long as religion is taught as a historical or cultural practice -or in later years, the Bible as literature and not as a religious text -it's perfectly appropriate and needed. Many children are never taught anything about any religion -ever. Part of understanding our world, our society, other cultures -even our laws is understanding how religion has shaped them. I'm NOT religious and have many problems with many religions, but I believe we all need a very deep educational understanding of them.
If you are looking for some type of church home or belief system to attend or become involved in with your son, I highly urge you to check out Unitarian Universalism. We love it, and they have a wide umbrella of beliefs. Basically we are people who see the road to spiritual peace as a lifetime journey where our beliefs and ideas have room to constantly evolve. There is a strong current of social justice for all and there is no discrimination. We incorporate and discuss many different religious practices and how they can relate to our lives. Have a look!