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. :/
I hear what you are saying and suggest that you set an example for yourself. You can take him aside privately and let him know the "way of the land' or simply make a class announcement that everybody is expected to work together on the tasks that Sensei sets before them. If that person is not willing to do so, he is simply asked to remove himself from class so that there will be no discord and that you will be more than willing to discuss your action directly with him after the class has ended and in private.
The other issue to consider is that the best wishes and expectations of others do not always exist in reality. The teacher is responsible for the WHOLE CLASS. No discord among the whole class can also mean removing a divisive element from the mix. Frankly speaking, I think that the Dojo Cho needs to step to the proverbial plate on this one and set the standard, because there is visible discord in the dojo. Just because it is not being directly confronted does not mean that it is not plainly evident for all to see.