[huntress]

One thing people have to stop doing is "humanizing" our animals. They do not think or reason in any way like we do. If he is getting aggressive it is our responsibility as caretakers to find out why and fix it if possible by studying them more closely not feeling like a failure and throwing in the towel.
You take a wild animal (essentailly)into the home its your responsibility to care for it. Something is irritating him and even though it doesnt seem like its his cage it still maybe the whole house! Another bird has come into HIS territory (his preprogrammed instinct) and took over and is luring away the attentions of one of his mates or family members. That will anger him. If you cannot bite the one you want then bite the one your with!!
You cannot blame him for being what he is.

The part about him "begging for scritches". Look at it as language. he is not "begging for scritches" he has learned in his time with your flock that the noise and sounds and postures he makes get certain results. Certain noises get you to come close and put your fingers near. He has trained YOU to come to a sound and posture. SO, he wants something. He is locked in a cage. He does what works, you come over, he gets to bite out his anger and frustration about the "new guy" in HIS cage. It had nothing to do with wanting scritches.

If it were my bird, I would begin to just let him out on his cage as with everyone else. Offer him lots of attention one on one (in the same room) with no touch at all. Move him around with a stick or perch only. DO NOT touch him if your not ready to take a bite. After a good bit of time with no aggression shown I would wrap my fingers with good tape or vet wrap in a skin color. Anything to protect them (fingers) to give you confidence. Offer him a scritch. If he offers the beak walk away. if he strikes - stand your ground and then walk away. Eventually he will want you. Try again and there will come a time when he will offer his head and wait until you scritch and then maybe strike (this is where the tape helps) stand your ground for the bite quietly. Dont back down btu stop the positive attention and then walk away. Only stay and talk quietly when he is not being aggressive - IT COULD TAKE MONTHS. But in the meantime you cannot disown him, yell at him, or shun him like a bad child or it WILL only make it worse. The best punishment for aggression is to walk away and not preen him. Preening (we call scritching) is bonding.