@Papatron I know, it's hard to block any inappropriate stuff on all the devices. My daughters rarely use the PC but when they do it's mainly in the living room and we can easily check what is going on. Anyway if his PC is Windows you can use the built-in parental control (windows family safety), add a child account (keep its password secret), set screen time limit, put filters on the microsoft browser and block the others ones.
It's not an infallible thing but filters works for the most and it's essential to prevent screen addiction. You can look at his web history and add web addresses to the blocked list... like Omegle.
We set also the dns on the router like advised here... idk exactly what dns is but this simple change worked very well: www.opendns.com/setupguide/#familyshield
Obviously keep the router password secret.
Years ago we used Qustodio on their phones, tablets and PC, but it started not working properly on the phones, so since it's not free, even not cheap, we switched to family link on phones and kidlogger (full version with fee) on PC. Now, for us windows Family Safety is enough, we still have Kidlogger free version installed but just for added security. You could try the microsoft built-in one and opendns, both free, if that is not enough for you, kidlogger free, paied or at last Qustodio or similar more complete parental control (with monthly fee).
Anyway, as @Meeeh said, a good talk is the first thing, you should be a good IT expert to avoid any inappropriate web stuff and probably a teen will find a way to bypass that too. I spent days looking for a good way to make Family Link non bypassable, it has been very hard even finding online the good guide that finally has enabled me to do that.
In any case the parental control apps can do a good job in limiting the most part of inappropriate stuff and mostly the screen time (that should never be more than a total of 2 hours a day), not infallible but still it's really worth installing them.