I agree that this is an HR problem.

That being said, pihole is great for this. Could even lockdown VPNs, or work entirely off of whitelists. It's a pain and constant whack-a-mole, but it can be done. Just not sure it's worth the effort compared to actual parenting.

Like, why does a 9 year old even have unsupervised access to this level of tech if he's being problematic? Physically take those away except for when he is explicitly allowed. If he needs a phone to be reachable, buy a dumb phone. Solve the problem on that level instead of over engineering the Chinese firewall.