For what it's worth, for those trans people that do not know their gender from early ages, puberty tends to be a resurgence of hormonal anomalies within the transgender body, as a child nothing may seem "off" because the physical feelings of being a boy or a girl tend to be not as different, genitalia is probably the only indication of sex in a child, but with puberty secondary sex traits begin to appear, and there becomes more incongruence with the mental state-body state.
It's actually incredibly common for people not to connect the dots until puberty and post puberty, and this is usually after those internal feelings of dissonance are given a name: transgender. I think most the transgender people I've heard of typically make these connections around teens or young or late adult (not having been given the chance to explore this in childhood, typically). Of those, some of those were more tomboyish/effeminate in childhood, and some were more closer to the gender roles of their birth sex. Every transgender person is different in experience. Those who don't match the "typical" transgender story are typically left out and faced with those boundaries pushed against them to invalidate their worth of being transgender, and that shouldn't be the case, because they need it as much as any other transgender person.
When it comes to gender, there is more to it than just genitals, the overarching combination of Chromosomes, Primary sex traits (genitals), secondary sex traits (pubescent growth), hormones, and neurological development is different in every individual, so when you get things that don't align or match like the typical male or female, you get someone who is gender variant or atypical.
I don't know how you son would describe it, maybe he had some feelings he wasn't sure how to describe or show at that age, or maybe he's equally confused as you now that he's an adult and he knows he's transgender without the typical childhood trans experiences, either way he's still valid and it's worth a talk. Your concerns are worth a mention, but not enough to hinder his transition. Perhaps see a therapist together so you both can voice your feelings. See where he is at, he may agree to wait until he has more of a grasp of his feelings, or he may be absolutely sure in himself and not wish to wait at all, you'll have to give him your trust if the latter, because at his age he'll know what he wants if he's absolutely sure of himself (I'm not entirely sure how many people detransition, but of the ones who are typically sure of themselves rather then questioning, it doesn't seem likely). Really it's up to you two what solution to come up with and works right for you.
And for the rest of us, thank you so much for being a good mother looking out for his interests, so many parents do not take the effort or courage to seek assistance when trying to be supportive of us. Thank you!
