Most transgender people prefer that you use their current pronouns even when discussing their past. If you don't know somebody's wishes and can't find out, this is a good default.
Important exception: never "out" a person as transgender without their permission. This can make them a target for violence and discrimination. Normally this is all the more reason to stick with their current pronouns, but sometimes situations arise where it's better to use their old pronouns.
As an example of how it can get complicated, one of my friends ("Bob") is a trans man (FtM). Shortly after he decided to transition, he took a job in a conservative workplace. All his academic transcripts etc. were still in his old name, which is recognisably female, and he didn't want his employer knowing he was transgender - so at work he had to present as female and go by the old name ("Kate").
Most transgender people are okay with talking about their past pre-transition as long as it's not going to out them. If you've been friends for years, you don't have to erase all that from history! Some people may have specific things they don't want to discuss, especially if they've had bad experiences, but it's not a blanket rule.
In this case, because he's your friend, it is absolutely fine to ask "how would you like me to handle these issues?" and he will probably appreciate you making the effort to find out his preferences.