Wait until he needs another job and then comes crying about “ageism” when it’s actually he didn’t keep up with the latest trends.
As opposed as I am to doing any side work, you better believe if I were in an environment that doesn’t allow me to keep up with latest trends, I would be playing with it on the side.
Before the shit show of the current employment market, I would be looking for another job if I saw I was getting behind technically.