If the amount of preening to his ears/wings is any indication, BumJu knew for a very long time indeed.

Spirits can be so smug sometimes.

Bumi sinks down to one knee as Aang rushes forward to hug him. The man has a black-belt in hugs, but only because he learned from the best: his father.

It's only after he's gathered Aang up in a hug that he brokenly whispers, "It's good to see you, Dad."