Tell him what you believe. However, it sounds like you may be unsure of what you believe or not confident in what you believe. If that is the case, perhaps this is an indication that you should search out and come to grips with this, as it will impact many other aspects of your life. This can be a time of discovery and bonding for your family. Your son will see how you tackle this and learn, regardless of how you decide to proceed.

Some believe that there is no metaphysical: no god, no heaven - just the material universe. This itself is a metaphysical truth claim that is not falsifiable. It takes faith in certain things to believe this.

Others believe that there is a metaphysical, but that it is impersonal and must be actualized through your actions or ritual. This is also not a falsifiable claim, and takes faith in certain things.

Still others believe that not only is there a metaphysical, but that a Person created the physical universe and has revealed Himself in history and in nature and that He may be discovered through examining these things. Also not falsifiable and takes faith in certain things.

If you are interested in exploring this last avenue, may I suggest reading:

https://www.amazon.com/New-Evidence-That-Demands-Verdict/dp/...