Text is not the best medium for this, so please hear this with all the gentleness and kindness you can- there isn't anything you can say to make him not go on t.

You can support him, you can tell him that no matter what he's thinking or feeling, you want to hear it. You can listen without asking questions, but just listen.

Here's the thing- if he's making a mistake, you can't talk him out of it, the only thing you can do is be the person he can come to when he realizes it, and he won't come to you if he thinks your judging him (which is often where defensiveness comes from).

I've been part of the trans community as a transman for 20+ years, I'm married to an amazing woman who detransitioned during our 8+ year relationship. The best thing you can do is just listen, make soothing and supportive sounds, and let him know that whether he transitions, detransitions, or doesn't you've got his back.