Ask HN: Tamper Proof Firewall for a Mentally Ill Friend

HN, I need your help: I have a mentally ill friend who lives alone. He is very high functioning, but also extremely mentally ill. He has some very... weird... things he looks up on the Internet. To be clear: nothing illegal! But stuff that child blockers won't block. It's very specific imagery that messes with his brain, but he's also addicted to it.

OpenDNS is out. He knows how to get around most things. He has physical access to his modem and router. So far the only thing I can think of is enclose everything but the Wi-Fi antenna in a locked metal box, but how do I lock down the firewall aspect of it so that it's similar to what he might find at a public access point such as a hospital or library, while I would maintain remote access so that I can control the software side of things?

The closest I've thought of so far is to force a VPN to a remote filtering proxy running on a VPS, but that's pretty much out.

Thank you for any and all ideas.