Well its up to you what you say since you know them best...
you might look up a brochure called "doh-transgender-experiences.pdf" . Only thing I would disagree with is page 7, where they state stress, instead many experience relief.
It states that being trans has biological connections, to do with development before birth which influences the sense of self.
It explains some of the feelings transgender people have.
You might think about showing.
His knowledge is simply outdated. And gender identity and sexual preferences are two different things.
hugs