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?
The situation is complicated enough (and your stepson old enough) that technical measures alone may not fix the problem.
Tell him how you and your wife may get into legal trouble if he chooses to do illegal stuff on your network. Ask him to come up with a suggestion on how to prevent this from happening. Who knows, maybe a discussion like this even helps re-ignite a bit of trust between the two of you? Deploying a network blocker without talking to each other certainly won’t.
You own the house, make it clear you don't want this on your network. Other than, open up to therapy if what you find is truly too far off and you think he is not good from a mental health standpoint.
Remember, your role is to help him. Not punish him or make him feel like a criminal. Connections are hard to make without being honest or upfront about the problem, it's going to take effort. Don't take the easy way out by avoiding to talk. If you damage the trust by snooping in or secretly doing something, then he is just going to feel more alienated.