I realize this is immaterial, but I'm curious. And I'm in no way saying that you're wrong. You absolutely have the right as a parent to determine what he does...
But my question is this: If he's tethering his PC to the phone, then he's not on your network at all, right? He's got his own connection, and unless I'm missing something, there's no security risk to your network. Unless you mean the possibility that he might catch something while using the phone tether, and then introduce it to your network later by tapping back into your network. That's a valid concern, I suppose.
If that's the case, here might be a simple yet creative solution... If he refuses to follow your orders and help safeguard the network, and he thinks he can get away with it because it's *his* hardware... then block him from using yours. Tell him to tether all he likes, but when his data runs out in a few days, he's SOL. Because he's not getting back on your network. Just an idea. Sometimes with kids who have decided they can do what they want... the best way to let them learn their lesson is to let them see what it's like without your help...
On the other hand... When I was around middle-school age, in my infinite tween wisdom, I decided that I was going to do what I wished regardless of what my parents said. Stopped doing my homework, wouldn't do chores, sat around playing video games all day. My mother and father had enough. They grounded me to my room for what amounted to nearly a year. (My parents had ironclad resolve.... I suppose that's where i get my stubbornness myself...) But not only that, but they took everything out of it. My TV, computer, video games, (we didn't have smartphones back then or that would have been included too) Everything was taken away. I had a bed... and a bookshelf. I was allowed to stock the bookshelf.
It was the best thing my parents ever did for me. I stocked my bookshelf with any and every kind of book you could imagine. Novels, encyclopedias, science journals, textbooks... whatever. And I read. I read and read and read. At first my mother was furious because I didn't seem to mind the punishment... but she was conflicted about whether taking away book was a smart move... so they remained. I imagine my IQ doubled in that year.
Now as an adult, I know that it was the best thing they ever did to me, and I've thanked them for it even.
Just an idea.