Don't add your 2 cents
Maybe "Don't add your 2 cents" is not the best way to frame this, but he is correct that being the boss means your opinion is dangerous. More accurately, casually tossing out half-baked ideas is dangerous for anyone with real power or even social influence. When your words carry enormous weight merely because you said them, you need to be more careful about the things you say because it will have consequences. If it really is just a casual opinion, and not something you have really thought about, it is better to err on the side of not expressing it in such a situation.