Yeah. I've got lots of them...
First of all. Severe Macs tend to have some pretty serious "one person bird" issues. And the trigger is usually, YOU'RE NOT MY PERSON. MY PERSON is in the room...
So, you could be the trigger for the biting behavior. So watch that one.
I've already got a "disfavored person" training post. And it kind of depends on how much effort your 9 year old is willing to put into it, consistently.
I would do the arms with towels wrapped in ace bandages and long sleeve shirt thing with your son that I've recommended before. And then do the disfavored person training.
You get the best results if you work with the bird twice a day, and work with him at a time when he is hungry. They are most responsive to food rewards then.
YOU pick up the bird and interact with him. Then you take him out of the main room, into a training room, with a training perch, and put him down on the training perch.
Then your son enters the room, and works with the bird doing basic step up/step down practice. (Rewarding the bird with praise and treats.) Contro the beak with two fingers, don't allow the bird to bite when stepping up.
Then you come in and take the bird back. Praise and reward the bird for being good with your son.
Once he steps up nice, THEN you work on the touching exercises I've talked about before. I use those wooden BBQ skewers. Your son uses those to touch the bird all over. He's not going to like it at first, but do it anyway. He'll get used to it.
Once he accepts that, you slowly work your fingers down the skewer until you are now touching him all over with fingers instead of stick.
Each time you get the bird and bring him back. This is all done outside of your presence. (YOU are the likely trigger.)
After that, once he generally accepts your son, it's time for "the test." Your son, carrying him on a protected arm, walks around the house with him, including the room with the cage, and you. (territorial/person triggers.)
THEN PLAY A GAME OF PASS THE BIRDIE BACK AND FORTH.
This is a 2-6 week project.
Do it at a set time every day. This is part of your routine now bird, until you behave yourself with the kid.
Once he goes to a second person, then, if you have others willing to risk getting bit handling him Do the "pass the birdie" circle game. (with treats.)
THIS is how you socialize and unsocialized bird...
