Thank you for taking the time to write and open-up regarding your son. You're on the right path, this is a great 1st step.
First off, tell him you love him and support him. He needs this more than anything in the world right now. This is the difference between merely existing (what it sounds like he's doing currently) but live and thrive in the current home situation.
Find a good gender-based counselor in your area, use Google, to take him to. He needs someone that understands these deep and painful dynamics of gender dysphoria, not just a regular counselor who doesn't fully understand things. Tell him that you're not trying to undo him into a non transgender individual, rather that you want him to grow and open up. A good gender therapist can help in this regard tremendously.
As a parent myself, I understand where you're coming from, but no need to feel like crap anymore, get in there, give him the biggest hug ever and tell him you're on his side every step of the way. And just listen...to his pain, to his challenges.... You're a good mom reaching for help rather than burying it any longer. Yes, a good mom! I wish my mom did that to me when I was 18, it would have saved decades of my internal anguish and hellacious depression.
Tell him there is no shame, maybe even point him here if he hasn't already found this place. A safe place to converse about all things trans gender.