I'm frankly amazed that ANY sensei tolerates that crap for a milisecond! A dojo is not a democracy. Whoever is that teacher on the mat is the boss in that moment. No ands, ifs or buts about it. If a student does not like it, then that student has the opportunity not to attend that class. When a student enters the threshold of the dojo, there is an obligation of students and teachers about proper conduct that should upheld. The teachers, particularly, the dojo cho, needs to immediately assert authority regarding proper roles in a dojo. You are already describing a situation where this student has undermined the working of the dojo to the benefit of nobody in attendance. I would have taken that student aside in private and spell out the way in which the dojo is run and leave it up to the person to either comply or leave, with NOTHING in between.
I agree with you 100%. If it was my school, I would of already had that conversation with him. Not to be mean to him or anything, just it isn't helpful to the junior students or class. I've done my best to control my own classes, while trying to respect that the Sensei wants there to be no visible discourse between anyone in front of junior students. :/