I'd neither agree nor disagree with his comments but have a dialogue about them. Disagreeing could entrench him or make him turn to others for support, who may encourage this belief. Agreeing with him could lock him into thinking he has to continue this belief without exploring it.
I'd say things like: Thank you so much for telling me. I'm really glad you did. It's important to be able to discuss things like this with the people who love you most - like me! Please tell me more.
Then I'd listen, and just ask questions very neutrally, without judgement. I liked to look and sound a bit puzzled when I was encouraging DC to work things out for themselves.
e.g.
So... are you saying that you believe only girls can prefer friendship with girls/love K Pop/ have a crush on Olly Murs/ wear make up?
Do you think men aren't men if they prefer sewing to football or like having long hair?
After listening for a while, just sound puzzled and say But some men are like that too. Show him pictures of famous fashion designer men like Paul Smith, famous rock stars with long hair and say, they learned it was okay to look and behave differently from the crowd. (Maybe don't add - without deforming their natural bodies, just be clear they exist, they are successful financially, romantically etc)
Or say things like... that's interesting. Can I share a different viewpoint with you? I grew up in an era where there was a trend called 'gender-bending'. That meant people liked to dress and look like the opposite sex, to challenge gender conformity. So here's David Bowie/Annie Lennox/Boy George (etc etc) They experimented with gender-role challenges without changing sex. Are you saying that you think we have to be gender conformist?
DS2 chilled me once when he told me that he is almost certain he would have identified as trans if he had been a couple of years younger when the trend really took hold in schools. He is very small (short) and pretty; loves fashion and art, hates sport, was convinced no woman would ever look at him because of his height and his lack of macho attitude to life. He is also autistic. He said he would have thought the only way he'd ever get lucky was to become a girl. And he was very lonely too, and would have been completely bowled over by the attention and camaraderie of the trans movement. Now he is past that adolescent excruciating self-doubt and self-loathing common to nearly all teens but especially autistic ones. He's happy in his skin, has had several girlfriends and knows he is a slightly built arty straight man and is fine with that..