Its possible, but not with the now router.
Having said that that even if you find compatale router and it lets you do that , your son may be able to get into it and change the settings .
II just tried a google search on how to block vpns on my router, and came up with this: https://community.tp-link.com/en/home/forum/topic/168528?sortDir=ASC&page=1
It might be better to look at other methods to monitor what your son is doing, things such as screen sharing where you can effectivly see what he can see, and access control where you can shut off a devices internet access based on time etc as well as throttling a connection which will slow things down to a crawl on a certain device stopping video/audio etc .
Fro example theres an app for TP lik devices where you can block or unblock devices as and when
As far as i know the Now router isnt capable of some/all of those but other third party ones are.
