If he’s living with you, you could try to block it at the router/firewall level.

On the machine, I’d just make it really slow rather than block it outright. Make it as unpleasant as possible to access those sites without blocking them outright. Making them slow makes it look like the sites suck. Blocking them makes it look like you did it or some conspiracy thing.

But if he lives with you, blocking them from your network is perfectly ethical imo