FWIW, my 8 y o has his own IBM Aptiva, his own Internet access, his own e-mail box, and I exercise absolutely no control over what he reads or says on the Internet.
You need to help him set up a pseudonym, Dmitri. Think about 10 or 20 years down the road when he wants to get a contract/ join a phyle/avoid an enemy or whatever, and his entire childhood e-mail and virtual-community history is greppable via a 2-second, 2-cent search. This would give his employers/ partners/competitors/enemies/governments a great deal of leverage over him in various ways. Also it might be very embarassing socially, which is no less important. Get him a pseudonym. You can always (more or less) claim the reputation capital that your pseudonym accumulated, but you can never (probably) lose history accumulated under your True Name. Of course he could always change his name/adopt a strong 'nym in the future in order to get a clean start, but this might not be easy, especially since people can find him through you and perhaps other of his relatives or friends. Sincerely, Zooko Journeynym