cjd, have no fear - it seems to be the case in the UK at least that most kids have nits (whereas when I was young it was rare).
If your son is not taking homoeopathic remedies, you can use various oils, whichever he prefers - tee tree, eucalyptus as Pat mentions, also rosemary, geranium and lavander.
However, some boys wouldn't enjoy smelling like that, and if he takes homoeopathic remedies, oils can antidote. So if you can get it, use Quassia chips - the bark of a south american tree. You boil it 1oz per pint of water for half an hour, strain and leave to cool. Then use as a final rinse and as a daily spray.
If your son is one of many at his school who has nits then a homoeopathic remedy probably won't help, but if he is the only one, or one of a few then it means he is unusually susceptible to nits and would benefit most by a homoeopathic prescription tailored to his totality of symptoms.