I was good at math and all sciences, but I am terrible at explaining things to people who dont think like me, so I probably couldnt help much as a tutor.
PQ and those offering to help, you keep mentioning how it would be difficult to help him over the internet. I CAN help there. Skype/Yahoo Messenger/AOL Messenger/ MSN Messenger, and probably a dozen programs I don't know would enable this communication via microphone and webcam. Verbally and visually. I would look into that.
PQ, you may have mentioned it and I missed it but, Is your son testing well but not doing the other classwork? or is he just doing poorly because he doesnt understand the material?
If hes not understaning, you seem to have willing tutors who would do better than I. If he seems to just not have the proper motivation I would be willing to talk to him. I was an underachiever in high-school and it has definately hurt me afterword. My wife was an over-achiever and she can attest to how much it helped after school. My wife makes $85-90k/yr, I make $36k...
We're both 21, so maybe he would relate better than he does to what his parents say.