What I really wanted to say would have been wasted on a boy who still sees the pursuit of sporting stardom as the most admirable chase in the world.
I wanted to tell him that I suspect McCullum has already mentally checked out of his test career - a difficult concept to grasp for an adult let alone a child.
I wanted to tell him that if it wasn't for the fact that New Zealand Cricket do not want Kane Williamson's first test series as captain to be such a high-profile one against Australia, McCullum could be retired to stud about now.
Yes, that would leave him one test short of that beacon of 100 consecutive tests, a phenomenal record that he will achieve when he leads the team on to the Basin Reserve on February 12, but that has always been a bigger deal to others than it is to McCullum anyway.
I wanted to tell Liam that one of his heroes looks knackered, but 10-year-olds don't really get that either. They're either wide awake and creating merry hell, or they're fast asleep.
They have no way of wrapping their developing minds around the concept of a deep, bone-tiredness that can't be fixed with a cup of Milo and a good night's sleep.
McCullum has no doubt been tired before, but it's been easier to hide it behind his pyrotechnic batting and bon mots.
Now it just looks forced.
You could tell that in Dunedin even, just one short test ago, when McCullum scored a rapid-fire 75 and 17 not out. A century was there for the taking in the first innings, it would have been his first for the year, but he tamely top-edged a part-time spinner to the man on the boundary.
In the first innings in Hamilton he deviated from recent script and tried to guts it out when New Zealand needed someone to steady the ship, but he was a man battling against himself as much as he was Sri Lanka.
The same in the second innings, the one we watched from the bank. He started slowly, responsibly even. But the madness kicked in, first with a forehand volley that fortuitously found its way to the cover boundary, then the wild slog that ended up nestling in Angelo Mathews' hands.
You cannot convince me the brain is getting the message to the hands and feet as quickly as it was a year ago, but how do you explain that to a 10-year-old? How do you explain to a boy that external factors in a complicated life can affect the ability to play games, when playing games is about the most fun thing you can think of doing in the world?
I could have explained to Liam how, no matter how tough the exterior, constant scrutiny and criticism ends up wearing you down. And criticism of McCullum has always been magnified and often unhinged.
Even on the way down to Hamilton for day three, we listened to a Radio Sport presenter opine that McCullum was no longer fit for purpose - this one innings after the aforementioned test where he scored 92 runs for once out; the sort of test that not too long ago would have locked you down a spot in the side for a year.
I could have also explained how being a witness in one of the most high-profile criminal trials in cricket history would have taken a mental toll, and that the disappointing tour to Australia would have added another level of fatigue on top of the exhaustion.
These are all the things I could have said to the boy, and some of them might even have been true.
But instead, I just said: "I have no idea what just went on there," and took a sip of my half-empty drink.
