He's expending energy.The nibbling isn't aggression as such, birds always want to be places they shouldn't and they don't understadn why you want to move them because they are comfortable. Calmly remove him and plce him on a playgym or out of cage perch. Don't react to the bite.
My bird LOOOOOVE feet. It doesn't mean he's been abused its just misplaced play. He's not meaning it in a nasty way its just his way of playing with you, he doesn't understand it hurts.
Try reading some of Michael#s training guides on aggression and biting- I really don't think this is aggressive biting though more displaced play.
