My boyfriend was arrested last night because he was driving without the interlock system in a car. They took him to Western, and is suppose to hear from a magistrate this morning... Worst case scenario ?

Depends on what else gets lumped onto the violation. Violations are problematic, the court has less urgency for a decision to be made. They are fine with letting the offender sitting for a while since they took the court's leniency for granted. He might end up with a slap on the wrist or he could be sent back to finish part or all of the remaining time on the original sentence.