Maintaining hand control is essential to having a well-behaved parrot. The parrot is not allowed on your shoulder unless you put him there. He should not be in control of being there. The parrot is not allowed on your shoulder if he has ever shown aggression while he is there. Watch his body language very carefully when other people or pets are in the room.
Some birds are OK on shoulders if their primary bondee is the only one present.
Don't expect him to behave on your shoulder. Don't turn his shoulder misbehavior into a game by rewarding him with drama. Please refer to the Pet Bird Report Back Issue Lists for specific articles which will help you with your parrot.