That's a great conversation starter! "Hey, isn't it interesting that talking about improving our working conditions is so offensive to the boss? Why do you think that is?"
If you're interested in learning how to form a union at work, you can pick any big union and contact them with questions. It might help if they're related to your field, but it's not a requirement. I worked with the Office and Professional Employees International Union (OPEIU) and they were incredibly supportive. Organizing a union at work is the best decision I've ever made.