Back to the OPs question, I think you need to have a very candid and non-emotional conversation with your son about whether he used AI in writing the essay. If he did, he needs to take the lesser grade or grades. He cheated and was caught and that is the penalty at this institution. Be thankful it is not more severe. If he did not, he should have demonstrable evidence that he didn’t including computer evidence of drafts, etc. Ask him to show them to you. Most computer software will have a way to look at the version history. Google how to do that if you don’t already know.
None of us are perfect. We all make mistakes and when we do there are consequences. Certainly this is a consequence but also a great learning experience for your child. I would view it as such.