S.S.
Sorry you have to go through this. I don't have a teen, but I was depressed at one point in my life and have a few clients that come to me for depression.
First you need to make sure you know where it comes from... the grades might not be it. The grades might create anxiety because at some level he might think that that is what is left of what defines him (since part of his identity as a athlete went away) and he holds on to it as an attempt to stay himself. But that's just a guess from the information you are giving.
Depression can be caused by different reasons. If his kidneys are not working properly he might not be able to eliminate heavy metals/petrol chemicals/ or other substances correctly and that can impact the way the brain functions and how hormones are made. I would check with a good doctor or even better with a naturopath.
You can also try hypnotherapy, sophrology. There are simple, non offensive methods that can really help him.
These podcst can be interesting to you:
http://itunes.apple.com/us/podcast/healthy-medicine-radio...
Good luck!
I see that you are in the Bay area. Contact me if you need references.