DH called me and said, "DS is calling you. BYE!" and hung up. At that point, I was on my way to the police station, so I waited around a few minutes and DS finally called from the GF's house and started out with some lame excuse that didn't even make sense. He said the guy whose number we had was wrong (he didn't know we called to check it, so major hole in that one!) and that he was "embarrassed," so he decided to just go spend the night in the woods so we wouldn't tease him.
That's what he said over the phone, but all I did was yell, "You just come home right now!!!"
In the meantime, DH called the GF's mother! She admitted to everything the kids were doing there. She let them have beer, but assured DH that there were no drugs. I feel so reassured.
Her DD is allowed to have boys over. She said that DS assured her that we knew exactly where he was. She couldn't have been any more apologetic, but DH made it very clear that our son is not to go to her house again and if he does, he would appreciate it if she'd call us so we could be the bad guys and take care of it.
He also called the police, who said that we could file civil charges against the mom. The state trooper said that basically, they'd all get arrested, including DS, go to jail until they could go before the magistrate, and then they'd get their punishment. If there's ever a next time, we're definitely going that route.
Also in the meantime, one of DS's friends finally called me back and she gleefully spilled the beans about everything. She said there was drinking and sex at the party. I believe the sex part, because he had the largest hickies I've ever seen ALL OVER both sides of his neck.
The girl I spoke with is a woman scorned, so she was more than happy to tell me what she knew.
I promised her that I wouldn't tell DS about her involvement, and I won't.
When DS came home, he got defensive and started making up his lies, but in the midst of him telling the tale, DH called to tell me about the conversation with GF's mother.
BUSTED! He got really subdued while I laid it all on the table. I was very proud of how I handled myself. I was calmly furious and there was no question how dead serious I was that this was the worst trouble he's ever been in.
I told him of some of his punishments, but let him know that DH and I needed to sit down and formalize them so we'd all be clear on what's what. Just off the top of my head, I did tell him that his social life is over for a good long time, he lost his cell phone, no computer, no TV. I also told him that his secretiveness has come to an end. I've put up with it for far too long, and that before he gets off his punishment, he'll have to start bringing his friends HERE and that I'm going to get to know each and every one of them. Only then will he be able to get out again -- but even so, that's in the future.
Regarding the secretiveness, I really hope this works. He is so closed off to everyone and has been since he was a child. It's going to be so hard, but I let him know that if he wants a social life, he'll behave like a normal kid and let me in his life. I also told him that IF I ever let him see his GF again, it'll be under our supervision for a very long time. I said that if their relationship can withstand that kind of scrutiny, then they can date some time in the future with my blessing.
That's all I can think of off the top of my head. I can come back later and let you know the details of the punishment. DH and I did this "four phase" punishment that gives him just a bit more freedom as time goes by, but if he blows it, he goes back to the beginning. It's all up to him how soon he ends this grounding, but it's at
least a month, if not far more. He would have to be perfect inside and outside of school to get out in a month, and I hate to say it, but that will never happen.
Thank you all so much for your words of support, advice, and encouragement. This truly was one of the worst days of my life and it really helped for me to be able to check in here and try to get my mind off the fear.
He had to share a room with his autistic little brother. Not fun, let me tell you. 
