The comb sold in Argos is also sold by Boots the Chemist its called a Robi Comb and is basically a nit comb which is powered by a battery with a small electrical charge that will kill any lice it comes into contact with. My daughter was badly infested a while ago and after spending a small fortune on varous preparations I invested in this comb, you have to persevere, coming morning and evening, as the buggers hatch, you zap them with the comb and after 2 weeks you should have broken the life cycle and stopped the infestation, if you then use it every morning and evening, when you comb you son's hair, any he may have picked up in class from his mates with less informed parents will get eliminated before they have time to lay eggs.
Also, using lots of conditioner on the hair makes it silky and more difficult for eggs to attach to, so when you wash your son's hair try using a good helping of conditioner!
I was also told, like Sapper, that Tea Tree Oil was good, but it is best used as a deterrent once you have knocked the infestation on the head ( pardon the pun) Hope you have success, I'm off for a gooid scratch, why is it talking about headlice makes you itch!
Cheers, Lucy