When I get home, I plan to write up my thoughts and feelings and give him a call.
I can't stand idly by while this happens to one of my favorite people.
Any advice on what to say would be appreciated.
Basically treat it like trying to convince him stop using drugs. It won't be easy and there is good chance this will fail for now or completely. You might want to look into if there are other family members or friends who will do this with you. Even if you talk him alone knowing who can help is valuable. To conversation itself.
First, do your research before hand. You need know what you want to say and how you want to say it. What he will loose if goes down this road, what he can gain by stopping and so on. You need know relevant vocabulary (cis, egg, dead name, gender euphoria ect.) so you don't get caught on semantics. When you talk you can be brutally honest but you have to be calm. You can show and talk about your negative feelings but you have avoid fighting words and other straight up aggression. Even if he gets agitated, you cannot response to him in same manner.
Two, pick your battles. There is only so much a person can take at one time. So choose your timing and topics. Keep it simple, consistent and don't get caught up too many off topics, even if they might need be addressed eventually. You can firm about transitioning being a horrible idea you won't support without going on about his other failures that maybe contributed to it.
Three, let him talk if possible. Ask open ended questions and have him explain himself. This is way he isn't just letting it in one ear and out another, but has to engage. Stuff can sound way better inside your head than coming out your mouth, is something that does wake people up.
Four, have alternatives ready to go. What he can do insted of his tranny crap and how he can immediately move himself away from the bad influences.
Five, you need avoid "I told you so" untill he's fully clean. Detrasitioning will be hard for him. He will loose friends, deal with hormonal changes and face inner demons. Most people hate admitting big mistakes and fixing them publicly so don't make it worse.
Six, be ready to walk away. I hope it works out but that's not the only option. He can choose the cult over you. Ultimately you can't change someone who doesn't want to change but you can avoid them dragging you down with them.