Easiest way to block certain websites during certain hours for one machine

[d0g]

I want to block some websites (discord, youtube) during school hours when my son is on his Mac for school.
What's the easiest way to accomplish this?
Haven't found any Mac software that will do it.
I tried setting dnsfilter.com -- but it's not working.

I'm on the verge of writing a cron script for the Mac that will tweak the hosts file ... but that will be a PITA.

Thanks for any suggestions.