If he can't get on board that the concept of woman isn't a feeling, which he very likely won't, since it won't effect him, you could just give him some examples.
Tell him that if self ID is adopted as custom, the crimes of voyeurism and flashing cease to exist.
It means that 14,000 men in prison for sex offences would have a legitimate reason to be automatically transferred to the female estate.
All women shortlists for Parliamentary candidates could be occupied entirely by men (he won't care because he's not a woman).
Tell him that customs and protocols are already being adopted.
Girl guides are now sharing overnight accommodation with boys, and parents are not to be told. This conflicts directly with NSPCC safeguarding protocols.
80% of tranwomen keep their penis and many are attracted to women.
Tell him about AGP. And how it is legitimately part of the trans umbrella. Cross dressing fetishists are now considered women. Men who fetishise women are women.
Ask him if he agrees with sex segregation. Because this will abolish it.
Tell him he is transphobic unless he adopts the term cis man. Because he has been redefined in relation to women who identify as men.
In fact, if he has no idea about the term cis, keep calling him cis and when he asks why tell him it's because it's dependent upon what a woman says.
Women identifying as men need to be able to have a new name for men.
if protecting the rights of women doesn't motivate him, maybe being redefined by a fraction of women might.