How do I convince my friend to not join the Navy?

Not sure if this is the right community to post this, but I'll give it a shot.

One of my very good friends is planning on going into the Navy in about a year. He's generally a moderate left leaning kinda guy, but is trans. He comes from a navy family which is part of the reason he decided to go in the first place. He's going into mechanics and stuff and not combat which is slightly better I guess, but it's very concerning to me that he's planning on joining especially because he's trans. I also feel that ethically, actively participating in that system is bad.

How can I dissuade him from joining? I don't wanna go full force argument mode because it's ultimately his choice but I would like advice on how to softly nudge him away from it without affecting our friendship negatively. Thanks.