I don't know enough about the life cycle and the efectiveness of hedrin. It seems to be the case that the nits build up resistances to products, so pharmacists change around (or should change around) what they are offering from season to season - what I'm saying is, an egg might have escaped or got lucky. or it could be reinfestation (it's not an infection, tgm!).
I know they're horrible but please try not to over-react -all kids get nits, mainly because some parents do no checking at all. So any child he's in contact with could be passing on the good news.
One of the reasons some parents say nothing is because they're scared of over-reaction.
Your child having nits is a fact of life - it does not show that you or anyone else is dirty, failing as a parent, or likely to catch anything else. They're just nits. Katchem and killem, as they said in Amityville.