We're going through this at the moment. The worst part is when we are in the car and Andrew decides it is squealing time. I can cope with the disapproving looks from the people in shops when he squeals, can even cope when he squeals through my phone conversations, but there is nothing like being stuck in a car on the motorway with a squealer!
I've looked through loads of parenting books and sites and all they say to do is ignore it. I figure that it will have to pass one day - I mean, he won't be able to hit notes that high in a decade and a bit anyway