I also started going downhill in 6th grade. I'm a girl and can honestly say for me it was totally driven by wanting to be accepted socially. It didn't work anyway, sadly. I did manage to eke out a degree and I'm a very happy SAHM, but I still feel some frustration that I could have done more with my education, given my intellect.
What I wish my parents would have done was make me toe the line. I wish they would have actually checked that I did my homework (instead of just asking if I did it) I wish they would have helped me to find clubs that I fit in with. They did help me by finding better schools for me, but that meant that I changed schools a lot. Once to a magnet in middle school and then to private after my freshman year of high school. Both changes were for the better, but they were too late socially. Kids establish their groups in 6th and 9th in the US and I was a year too late with both changes. I think that's important.
I can't speak for your son, but what I wish my parents would have done would be to be obnoxiously involved on a daily basis. They were very detached. That approach didn't help me.
