It's *only* a minhag...?

A very disturbing occurrence happened last night.

We were lighting the menorah for the very first night of Chanukah. I our home - as in many households all over the world - we made the blessings and began singing the various songs associated with the lighting. in the middle of Ma'oz Tzur we have a brief interlude by the passage of "Yevanim" where we change the tune from the regular singsong to a more festive faster paced melody and we dance in a circle for a few minutes before continuing with the standard tune.

For some reason, my eldest son sat out the dancing, and didn't participate in the singing altogether (he's six years old). When I asked him about it, he dismissively gestured and said something along the lines of "it's only a minhag."

Admittedly, that was not one of my finer moments; I got very angry and responded to him harshly. Where had he learned such an attitude? Where did he even pick up such an idea? He certainly didn't learn it in our home and I don't know where he could have picked up such a sentiment. Of course I could see that he regretted saying that immediately in the face of my response but more than the isolated incident itself I was concerned as to where he could be getting that from. I was beside myself.

Later, I tried to sit with him and explain that what sets us apart is not only our adherence to halacha but our respect and in some way fealty to minhag and mesorah. Even the chag of Chanukah itself is set apart from the other chagim because of it's inextricable ties to mesorah and communal consciousness, more so than any other holiday. How do you explain to a six year old that the real reason why we do things is so much more transcendent than the fact that it's all written down and laid out in a convenient code of laws and instructions?

Yeah, he's only six. But that concept didn't materialize out of thin air. Am I somehow demonstrating that the "way it's been done" isn't good enough or is somehow lacking?