Alan "geeked" himself into a corner trying to protect his family on the internet. Leo says that OpenDNS is the best option because he can block unwanted websites from the DNS level. That prohibits teens from getting around it. He can change the DNS settings to OpenDNS at the router and it has great filters he can use. Then he can password protect the router so they can't bypass it.
Of course, the kids could just reset the router, but then he'd catch them. If they have a mobile phone, they can simply surf via 4G and they're out in the open again. SafeKids.org has an internet contract and he can get them to agree to behave online or suffer the agreed consequences. Perhaps sitting them down and talking to them about the realities of what's out there is in order.
How about Mesh Networking for WiFi issues? Leo says that the Plume works if he can get it. He should also look into the Eero, which has some additional smart features for disabling internet access to specific devices. Another option is Cujo.
(Disclaimer: Eero is a sponsor)