Your son is still your boy, your child cannot change sex. That fact is going to be very hard for him to figure out how to live with, but, it is true.
It is ok to grieve for the name you chose for him with such care, and the life you envisaged for your child and to feel confused about this entirely new person that they say that they actually are.
I hope your adult child is able to find support to manage their way through this.
Have a look at:
Have a look at genspect.org
Stella O'malley's podcast "gender through a wider lens"
and (at) CAN_SG on twitter
and have a look at (at) Tullip on twitter, there's a couple of threads about him on FWR. He's about to sue his gender clinic because he does not now think he should have fast tracked to transition.
Affirmation is, according to Hilary Cass, "not benign" and so I'd be asking for differential diagnosis to rule out other reasons for your son's gender feelings. Make sure he's not neurodiverse, struggling with sexuality, has a history of trauma or any of the other causes of sudden gender issues - because none of those causes of gender issues respond to transitioning treatment.
If he is actually gender dysphoric then be cautious about medical or surgical interventions. As Tullip's case shows, they aren't all that.
Good luck. You'll manage, you love your kid.