Good thinking but I don't reckon he's a metamorphmagus nor do I think someone was watching over him.
I think his hair just regrew because he's a wizard.
Remember in the first book Harry said to Hagrid that there must be a mistake about him being a wizard and Hagrid said something like, "Never made things happen when you're upset or angry?"
I think that's all it was. He was upset about how he'd be teased with his hair like that, so since he's a wizard, it grew back.