if he is a member in good standing of his union then (if canadian law is simular to american law in this area) the union has no choice but to represent him in what is essentially a dispute over firing.
he won't win, being drunk at work, bringing a gun to work, threatening a boss, would all be "gross misconduct" at the very least.
the union has to file the papers and provide him with representation if it goes to court, they have no choice ( in america the union would be in violation of fair labor practices by picking and choosing which dues paying members will be represented and which are ignored)