Get him to start by saying that to make sure you’re not talking at cross purposes, you want to know what he understands by the terms man, woman, male, female, sex, gender, transition, transman, transwoman.
Use questions to make sure that you (ie he!) really understand what he means - when does actual transition happen, what is it, does it happen once in a flash or is it slow and drawn out, what about people like pip/Phillip bunce who are female approx 3 days a week, how does it relate to having a penis or not, how do sex stereotypes tie in, if a girl cuts her hair short/man grow his long does that make them trans (in which case explain Princess Diana et al with short hair/a load of long haired male iron maiden fans), etc.
Also - how come a woman can’t change sex enough to inherit a male title or money or become a priest so are there different types of male but not female?
Also - hypothetically (depending on his current relationship status!) - if he was swiping through on line dating profiles - would he expect to be told ? What level of ‘trans’ ness would be acceptable to him? And so on...
Just getting into those questions in a discussion will begin to lead to everything else. There’s a reason why TRAs etc are so keen not to let anyone define what the word woman means - because as soon as you do all their arguments crumble around them.