It's not your decision, you need to respect your friend's choices, even when they're against what you would do.

Best you can do is sit him down and state your concerns, but unless you're willing to throw away the relationship, you have to make it very clear up front that you're going to stick with him no matter what.

Also, while there is always the chance of getting stuck in a bad situation, the military is extremely open minded with respect to the trans aspect. The upper leadership has made that very clear to Congress. Might want to make sure your assumptions are correct before you start the conversation.

I was 0317. Attached to a lot of low drag units in Iraq, Kandahar, Syria, elsewhere. I regret it. However everyone's service is their own.

Ask your buddy to ask his recruiter how they define non service related injuries. Find out what your friend wants their MOS to be, look up common VA admissions for that assignment/job/task.

What are his goals after service. Why is he joining up, not why he should, but why he wants to.

See if he'll go to a VFW and talk to some vets and ask what they did, what they regret and what they miss. Make sure he opens with "I'm thinking of joining up". Make sure he pays close attention to how he's treated, the brass may say it's a trans friendly service, but there's still rampant racism in the ranks and we've been desegregated since WW2. You'll see what the military does by asking veterans.

Don't push them away if they join up, if shit goes wrong, they'll need you. Don't be a shitty friend. But definitely do let them know in all honesty how you feel, you're not going to convince them to quit. They're gonna do what they're gonna do and you need to trust them.

I'm sorry, there's nothing you can do. He has clearly made his choice and the only thing you can do is react.

You could share this, and also express your concern for his well-being and love for him as your friend, but I agree with the others that you can't steer someone's choices.

I had a friend join the Canadian military recently, and while I disapprove of his choice, I can't control it.

I'm a recovered addict/alcoholic, and while I have a general dislike of groups like AA or NA, there's a saying that's stuck with me: "Didn't cause it, can't control it, can't cure it."

It's up to you if you choose to stick by him. Personally, I would still want to love and support him.