Attention screaming. He probably hasn't been getting enough attention until now. This usually subsides as the bird gets settled in, and gets his attention needs met. I said USUALLY... sometimes it gets worse, and then you need to do screaming behavior modification.
Bite pressure training involves beak play with the bird on your lap, communicating when the bird is applying too much pressure. (If he is applying way, way, way too much bite pressure, I generally use one of those rope dog toys.) If he latches on to your fingers, or your arm, or what have you, stop the game and tell him "that's too hard."
You can control the beak of a macaw with two fingers. The beak may be powerful, but that scrawny little chicken neck isn't... right now I would. When you can trust him, then stop holding onto the beak with two fingers.
If he is biting, he gets dropped to the floor and has to sit there a minute being ignored. He gets forgiven the minute he steps up nice.
This bird also needs to get used to being touched all over...
Finally, take your time, go at the bird's pace.
My Greenwing macaw hadn't been handled in 8 years... It took me 4 days to win her trust. She's been a great bird ever since!