As previous posters have said, maybe best to let the dust settle for a bit.
Then maybe start pointing out stories in the press where eg convicted male paedophiles are being referred to as women, or men who have been in the military are kicking the crap out of women, for entertainment, etc.
Does he know the statistics? Does he know that 98% of all sex crimes are committed by men, not women? And that's why we segregate by sex. Does he know that one girl per day is raped in school? And does he know that 90% of all sex attacks in changing rooms occur in those which are mixed sex? And does he know that the overwhelming majority of transwomen, about 90%, keep their penises and many are heterosexual. And that transitioning doesn't change those statistics, in fact, given the loopholes afforded by transition, it increases them.
Sometimes it's a lack of knowledge about why women need separate spaces in the first place that lead men not to get it.
A lot of people need a fast track lesson in feminism before they understand. And obviously, in a fairly sexist society, that hasn't been happening.
At some point in the future, you could also explain that there are two types of transwomen.
This excerpt from Helen Joyce's book Trans, should help. You can tell him she is a journalist for the Economist, if he's already suspicious of her credibility as a woman...
quillette.com/2021/09/07/the-truth-about-autogynephilia/