Need help with my 19 year old

My 19 year old son announced to me today that he wants to change his gender. He does not show or has shown any gender confusion, no tell-tale sign of feminity traits. He has never had female intuitions or have been or played/acted out in a "female" role. He has had a girlfriend for 4 years and they have been overly sexually active (explorative), but he has never explored or had crushes on his guy friends. He has no intention of leaving his girlfriend, after he goes through hormone therapy, and doesn't want the full surgery. He has stated that he looked up transgender information and he thinks and feels like them. He tells me that he feels like a woman inside but can't describe in what situations or why he feels that way. I have known trans gender and gay people throughout my life and have impathy for their own private hells and have adored them for their honesty and courage. To gage my son against them, I don't see or feel it. Which I love my son no matter his preferences or his personality.
I know he suffers from depression, he isolates himself, he spends hours and hours on the computer, he is unkempt, 2 years behind in maturity level, and has no discipline to complete his intended goals.
I am the type of parent who is open and honest, but this is a bombshell landing in my lap out of the blue. I am wanting him to have therapy and to talk through his immediate issues then with clearing away these issues then make decisions about his gender.
My inuition is telling me that this is more of "shock and awe" teenage rebel yell. But I do not want to dismiss this if it a more serious mental illness or really his "personal knowing".