Ask HN: Best practices blocking an estranged son from accessing illicit material

Background, I have an 18 year old old stepson that has been estranged for 2 years. A few months ago, he came back into our lives. He's in college, but on winter break. Tonight, he was visiting his grandparents and left hos computer in my (and wife)'s home unlocked with, well, questionable porn on his desktop. It was anime, but definitely depictions of minors. Without scouring his computer, it does leave me as concerned.

I have no control over his computer (he's an 18 ur old emancipated minor, but back livong at home).

I do have control over the network to the outside world. I've been thinking a Pi Hole to sink his perversions into oblivion, but this will obviosly be a reactionary.

I thought about backdooring his PC, but it's Win10 home, so doesn't support remote access without tools I dont want to get into (was hoping Pro and enabling RDP, but, no).

I can easily change the Wi-Fi password, but my wife insists I dont as she doesnt want to alert her son that we know.

We dont care as such if he has normal porn proclivities, but given his mental history and actions, we want to be able to block him from illegal activities, especially on our network.

Any suggestions?