Restricting internet browsing in wifi login portal popup. Disabling Safari doesn't solve problem

Sounds like he found a viable work around. Smart kid.

I don't know of any other way to restricting access other than outright disabling it which is what you've done. He's managed to find exploit something to still gain access to the internet, and if you're trying to limit that you might just have to downgrade him to a burner phone like a flip top phone or something of that sort if he can't be reasoned with.