As much as the bleeding hearts hate to admit it, there has to be a line somewhere between freedom of expression and sedition. At some point you have to say "Wait, he's advising soldiers against who oppose the war to do so violently."
If this isn't that line, I'd hate to see what is. This should be a law enforcement matter, imho. Not that I think it would happen, hell, I don't even think he'll get fired.
And before people start yammering about oppression and the COnstitution, I'd advise them to think back to the Revolutionary war, and consider what would have happened to an educator who openly advocated soldiers murdering their superiors.