I wouldn't worry about it too much, as long as you are trying. You've known your brother as a 'she' for much longer than as a 'he', so it will take a bit of time to re-train your brain.
It took me probably about 2 years to finally stop calling my brother "Jeffy" all the time. I had called him "Jeffy" for 7 years, then he no longer would let me use his "baby name", and wanted just "Jeff" like his friends all called him. It never felt natural to use Jeff, so I call him by his full name "Jeffrey" all the time now!
So don't feel too bad! I had a hard time with a simple change in variation of name! I'm sure he won't be mad at the occasional slip, as long as he knows you are trying! And just make sure you try REALLY REALLY SUPER hard when around people that don't/may not know his past, as to not accidently 'out' him! Never a good thing!
Talking about the past is an iffy subject, I'd say use the current name and pronoun unless he does differently in the situation. It would probably be best to discuss it with him. You also may need to watch which stories you tell to who, again you don't want to accidently 'out' him to someone that doesn't know his past. Who he tells and when he tells them should always be left up to him!
Good luck to you and your brother!