I was thinking the same thing... My son wasn't "officially" diagnosed until he was 7, but i saw ALOT of his tics when he was very young. Eye blinking was the worse. I had to chuckle when you said your son greeted a friend by humming Motzart, Scott use to walk around humming stuff from Vivaldi. He is very talented with vocal music as well. I too know nothing about PDD-NOS. I hope you get a positive diagnosis soon so you can know exactly, what it is you are dealing with.
Vicky