Not that it's any of our business, but why would you want to prevent your son visiting facebook on (presumably) his own computer? It's tipping into a behavioural issue rather than technical.
Because it spreads viruses and trojans, as I discovered to my cost. His computer operates on my network, which I use professionally, and I do not wish to take the risk. I have no concerns over bandwidth.
I appreciate that FB may have valid uses, but I fear that it is turning a generation into social cripples who believe that they have ´friends´via a virtual medium of communication. Fortunately my son has a normal social life, goes out, plays sports, and has real friends, but I know others who spend every waking hour on Facebook and it is most unhealthy mentally an physically.