Dear L.,
You've gotten some great responses from people with much more experience than I have, but I wanted to add or maybe emphasize a few things.
First of all, good for you for being so concerned and trying so hard to figure this out! It sounds to me as if your son is shutting down. I was a junior high school counselor, and I saw kids shutting down (stopped working, etc) when they felt overwhelmed, for whatever reason. I'm seeing my junior high and high school age nephews do it now. It's very frustrating! There may not be an apparent reason, and you may never figure out the "reason." I think depression is a real possibility but not neccesarily it. I admired Elizabeth's honesty....and I think the whole sexual thing for HS'ers is too overwhelming, whether because of getting sexually involved or not getting involved and being thought weird.
Whatever it is, I would switch the approach to a more matter-of-fact one. First, express to him your great concern, and your great pride in all he has accomplished thus far. Then ask him what he needs. A change of school? Lighter course load? Tutoring? Try to engage him in finding a solution. My sister is using Sylvan for her JH'er; expensive but effective.
I agree with the mom who said too bad we don't have vocational training anymore. I think even the brightest boys need more "manly" activities now, where they are challenged to use their physical strength and their male brain (!) to work on things...plus, it helps them if they can be around some tough (not mean) men - they just need that. And girls have needs that aren't met by current public ed, too. Maybe getting him into karate or finding a community college course he could take would motivate him.
I definitely wouldn't take away the opportunity to see his sister, and I could tell you don't want to do that anyway. That's the desperation talking! This is such a hard time for parents and kids. I loved all of Elizabeth's advice (and everyone had great input), so I will just say, don't give up and don't despair. Even though it's HS and seems so crucial to get his educational act together, the bigger picture is you want him to know you are always in his corner, no matter how "bad" he's acting/doing. He may already be anticipating the day he's out of school and more on his own, and that can just seem very scary. So if he doesn't move forward, he won't have to deal with it. What you and he and all of us need to trust is that we will have what it takes when we need it. If he doesn't have it right now, be patient and supportive. Don't give in and give up, but don't get too polarized with him on all this. Wow, L., I hope all this helps and I hope you find the right answers for both of you. Again, he doesn't sound like a bad kid at all; just overwhelmed. And it makes parents mad to hear that sometimes! When our kids get so much support and have everything going for them, what do they have to be overwhelmed about? But just trust that his "inner self" knows more than he consciously knows that he needs to put on the brakes and stop performing for awhile....and give him time to find his answers. Again, not to the point that you enable him to never move forward, but try to think more like you're his coach or trainer and he's recovering from an injury. He needs patience AND pushing....freedom AND boundaries....love AND toughness.
God bless you and him both! Have a fun family time over spring break and let him know how much you love him.