Thanks everyone for the great responses so far. Keep them coming.
I feel horrible about what he is going through and I just want to wrap my arms around him and hold him there forever.
I think this is your answer. 6th grade is a really difficult time, friends, social interactions, and finding who you are.
I get flamed for this, but as a mom I have to say it TAKE HIM!!! FOLLOW YOUR HEART!! You are his mom, and if you feel like you want to grab him and hold him, then this is the opportunity to do it. What better place to bond, talk, and really get on his level, heart to heart with him. A night out on the town isn't going to do it. But a weekend away conveys to him, that you care deeply about him and that is what he needs.
You don't have to explain to your other children where you are going, being "fair" is overrated. Each child is different and needs different things. In your case, your little one needs you and this trip could jumpstart an opportunity to open the lines of communication when they need to be opened most. Saying that everyone should get equal treatment in a family, and taking one to disney isn't as good as hunting with dad, etc. is not the issue.
Your son needs you and I think would benefit greatly from Four uninteruppted days with you at a magical place to uplift his spirits and to give you both time to talk. I wouldn't tell him where you are going, but I would tell him that you want to spend time with just him b/c you love him and care about him and want to talk with him.
Go with your heart, not your head. And as for seeing it as a reward for bad behavior, address this head on that it was one of the factors that made you decide to take him, that he seems to be struggling, but this is in no way a reward, but instead a hand of grace extended to him b/c you love him and want to sieze the opportunity to get to know what's going on in his life.
TAKE HIM, YOU WON'T REGRET IT!!!!