Your boyfriend is legally entitled to make workplace complaints to his/your state's Occupational Safety and Health Administration (OSHA). When I say legally entitled, I mean that he is able to make a complaint and be legally protected from retaliation from his employer for making such complaint. Generally speaking, I do not believe that OSHA will disclose his name to the employer, i.e., OSHA will not disclose his name and that he was the one who made a complaint.
In terms of "throwing him under the bus" if an accident were to occur, I mean your boyfriend would be legally responsible for any accident that occurred if it was his negligence that caused the accident (speeding, etc.). However, the employer would also be held vicariously liable because your boyfriend is their employee.
In terms of "throwing him under the bus" as in firing him for being in an accident, generally speaking businesses may fire an employee for any reason so long as the termination is not due to your boyfriend being a part of a protected class, i.e., the employer cannot fire your boyfriend for the sole reason that he is black (hypothetically). Therefore, in this sense, the employer could "throw him under the bus" in the sense that they have the legal right to terminate him for any reason whatsoever (as long as not for the sole reason of being in a protected class).