I may be out of line here, and I certainly don't mean to be judgmental, so just take this with a grain of salt...I only want to help you. He told you he was embarrassed and didn't want to tell you he was having a hard time. But...do you think that possibly, given your reaction to forgetting his books, he might be AFRAID to tell you, because he fears what you'll do if you see bad grades? From his perspective, when you only thought there was a POSSIBILITY of a bad grade, you grounded him from everything and threatened to take away sports. And that wasn't even because he actually got a bad grade.
I was a really good student in high school...almost always a straight A student...I got a B+ about once a year if at all. But my mom would freak out about an A-. So when I got behind on my math homework, I didn't tell her because I was terrified of what she would do. Consequences were REALLY harsh, so it didn't really invite confidences. If she made a huge deal out of something tiny, why would I go to her with something big? I couldn't trust her not to overreact and be helpful. And now that I'm older, of course I understand it would have been better to go to her BEFORE it got out of hand, and that she would have done her best to help. But as a kid, you can't always see around corners....you don't know what to expect, but past experiences tell you it's going to be bad. He doesn't want to disappoint you, and he equates struggling to failing, which will disappoint you. I'm betting that he's thinking you expect him to do it all easily and the fact that he is having a hard time makes him worry that you will be mad at him and think he's not doing his best. He knows what you expect, and he knows how you have reacted to his failures in the past. He knows you wouldn't be happy about those grades...but how can he admit he needs help without telling you about the grades that are probably going to disappoint you and get him in trouble? And you don't have to answer this....it's more just something for you to think about: If he had brought home those grades when he got them, what would your reaction have been? Would it have been to offer him help immediately, or would it have been to tell him how disappointed you are in him? "It looks like you are having trouble with Science. What can we do to get this grade up?" or would it have been more like "How could you get this low of a grade? We studied this!"
I totally understand the worry, from your perspective, but our parents' reactions kind of show us how serious something is and how we should feel about ourselves. He was probably already feeling bad about himself, worrying that he wasn't smart enough. My mom's reactions never motivated me to work harder....I was already doing my best, staying up late to get homework done, and I would stress myself out so much for tests that I would completely blank and I wouldn't sleep the night before a test. So it never made me want to work HARDER....it just made me feel like a failure and it made me afraid to tell her anything. My friends didn't understand why I was in tears over a B+ when they were excited that they got a B! Again, I don't know you, and I wasn't there, so I don't know what you said or how you said it, so I may be way off base. It's just something to think about, because I come from the other side of it.
