I find it the weirdest thing to have added to the dash display. My Camry has one. It went up to around 26, the promised mileage, as I first drove it around. It's been there ever since. I suppose I could reset it every now and again. If my mileage ever dropped off discernibly, I suppose it could be a sign that something was sub-optimal in my engine performance and I should have it checked into. But it strikes me as hardly worth whatever it cost them to include it for 1) just to prove to me that I'm getting the mileage they said I would and 2) the rare case where a dropoff in mileage is my only sign of some repair need.
So, no, I don't think you have to eat your hyperbolic nonsense until your son ever again in the future of his interactions with you again once mentions that gauge. Because I predict he won't.