You can install a proxy on your box (there are several free ones). Then make your browser to use the proxy and block the undesired content at the proxy.
It's not the best idea, but at least it will make your kid have to work a bit more to disable the protection. And if you want make him work a bit little more, install a firewall and block all the IPs of the servers that provide porn (for exampe 209.247.216.23 and 209.247.216.24 for blocking danny.com site).
If your kid disables the firewall and the proxy in no time you should buy him a book about network security and network management, perhaps you have a little h4x0r at home. Do not blame at him, teach him. Perhaps he will be your next security and network admin... who knows
