When you were a kid wasn't the funniest part modifying the design or building your own? Isn't that kind of the point of Lego? It's in small blocks so you can create something with your imagination?
He gets new kits and starts by following the instructions. He learns techniques along the way. How to interlock to make things that are strong and won’t fall apart. Patterns for building subassemblies like body panels or spoilers.
As soon as he’s done following the instructions he takes the whole thing apart, and starts building his own creation with the pieces, applying the concepts he learned.
It’s kind of like music. You don’t learn to play piano by composing songs, you start by playing other people’s music to learn the fundamentals. Then you can start riffing on it.
There’s also kids who need a bit of push at the start, and/or will go through a full build following the directives to see what they want to change, or how they’d build it better.
TBH I fall in the latter category, and take that approach for a lot of things. I.e. I’ll follow a coffee brewing recipe to the letter to see what it gives, and infinitely iterate from there, including taking completely different approaches to compare.