How to convince my supervisor that he is wrong?
The whole point of scientific debate is to refine our knowledge. Go into the discussion with a mentality to learn, not to win. Keeping this in mind will help you keep the discussion not adversarial and not emotional. On unresolved disagreements, try to find a way to falsify either hypothesis. If it is not possible, consider that which option is correct is probably not that important.

Listen to what they have to say about the criticisms, maybe they disagree with them, maybe they agree (and maybe even that is why the brought new people in). Probably, it is somewhere in the middle.

If (or when) your relationship is good, and you have a good understanding of their position, you can even propose playing devil's advocate.

Of course, I am assuming that they are decent people and not tyrannic professors. You should assume the same until proven otherwise (and if so, consider if you want to shut up and obey, or walk away).