Usually in a union, there's a designated mediator available to assist with such problems. Workers are also protected, generally speaking when communicating their "protected" rights under state or federal employment laws.

If there isn't a union mediator, your boyfriend feels comfortable with. He should schedule an appointment for a consultation with an employment attorney. During the interim he should save and/or ask for copies of anything he's asked to sign. He can also respond in writing on whatever he's asked to sign. If not already, obtain an employment handbook from HR, or talk with an HR representative.

Those three items are about all one can do.