It may seem, on the surface, that nothing matters if there is no God. However, in my opinion, I think everything matters more if there is no God. We have no invisible "creator" we have to pay homage to; we only have our fellow humans and living creatures to cooperatively live with.
The absence of God takes away all justification for arrogance and entitlement that anyone can espouse about themselves or others, and it truly puts us all on an even playing feel where the only thing that can affect the outcome of our lives are our experiences and the experiences we give others. In reality, then, everything matters even more so because this life is the only thing and only time we have to give our children and their children a better world, and our ancestors are the only reason we have the world we have today. The way I see it, we're all even more interconnected than we could ever imagine - all we are and all we ever will be is a result of the people and creatures who lived before us. I find great beauty, comfort, and hope in this giant "hand me down" scenario we call life. It's both humbling and empowering because it truly demonstrates how we are all connected and what our true goals should be in life. I hope your son can find comfort in this view as well.
The opposite is true. Since this is the only life, the only time we'll ever have, there is nothing that matters more. The only "afterlife" that's real is in the memory of the people whose lives you touch while you're alive.
There are books that you can read that will help you respond to the "without god there is no meaning" claim. Erik Wielenberg offers a good one in Value and Virtue in a Godless Universe. Andrew Kernohan offers another A Guide for the Godless: A Secular Path to Meaning. Dan Barker wrote one you might find helpful called The Good Atheist: Living a Purpose Filled Life Without God. Kernohan's book is actually available on his site as a free pdf download. Wielenberg's book is available in the Think Atheist Radio Amazon store, as is Barker's. Wielenberg's book is a philosophical response to Arguments from Meaning and Morality while the other two are more discussions of how to have meaning without a belief in god.