One way to approach the topic in hand is through describing the scientific/materialistic mindset. Knowledge is divided into deductive and inductive, or reason and inference. Truth is what is reproducible through time by artificially separating reason from feelings. This act of separation serves to avoid reducing what is true to how we feel about it considering that the observer himself continues through time and he is ambiguously aware of the unreliability of his feelings.
Through observing reality using inferences, the lack of ultimate truth is known through associating phenomena with conditions and causes. As such, the meaning of this endless change becomes the ultimate truth (in disguise) as it seems to endure the test of time (we are often encountered with the notion: the only constant is change itself).
In this state of affairs, the artificial separation of reason from feelings as well as the apparent lack of ultimate truth (and therefore purpose) becomes the basis for justifying behavior that does not take morality or social norms seriously (moral nihilism). This is why most nihilists are hedonists.
And yet, you can find nihilists who take morality seriously, but they would make sure to justify their actions based on humanistic values, not on the backward religious superstitions according to their views. This kind of nihilism/materialism would often conceive human achievements in arts, science and technology in very high regards. Letting go of the old and outdated belief systems becomes a driving force for human progress that has no limits where the human utopia can be eventually achieved, and where accepting a world without purpose (except what we decide) becomes a sign of maturity can we can decide on how create our own future. In this mindset, it is not uncommon to encounter notions such as "choice is everything" (as the shadow of interpreting phenomena through determinism)
More generally, the best approach to discussing philosophy with your son is through a dynamic conversation, especially when the topic is nihilism
The cleverest defenders of faith are its greatest enemies: for their subtleties engender doubt and stimulate the mind. -- Will Durant