See if you can gently encourage him to run a one-shot - or hell, even to just read the CRB - of a narrative-first improv-heavy game like The Sprawl [1]. There’s a good chance that would widen his horizons considerably, maybe challenge some of his assumptions about what a GM should and shouldn’t do.
[1] a cyberpunk-themed Powered By The Apocalypse game. I just ran two sessions this weekend. It’s good!