If the second explanation is the case then you need to balance the messages you send to your son. (A good idea to keep in mind no matter what the state of your teen-parent relationship.) In general, parents need to be experts at catching their kids being good, which at this stage is more difficult than catching them being bad or disappointing. A quick exercise.
- Think of three things you love about your son.
- Think of three things that bother you about your son.
- How equally do you communicate these things on a daily basis?
Obviously, the idea is to stay balanced between good and bad with your teenager. It is necessary to do this for yourself and your teenager, because no matter what it looks like, he is hearing what you say.
Now let's look at the second essential aspect of this question--before we talk about a possible solution--the difference between what he feels and what you observe. You see a logical connection here: his slacking off leads to poor grades and poor overall school performance. As a parent, part of your job, like it or not, is to hold your son accountable in a consistent manner. Thus you ask him to explain what is happening, so that you can understand and possibly help out.
He knows what you are asking, but believe it or not, he does not understand himself what is going on. All he knows is that right now everything is confusing, and he has lost his way. Worse, he is scared that he will never find his way again. For the first time he is doubting that he really is intelligent, and is too afraid to try hard to find out, because he might learn that he is not smart. If this happened he would be devastated and you would be disappointed in him, which would further devastate him. Plus, we cannot forget all the physical and thinking changes that are happening--hormones demanding attention that he cannot escape, and the emergence of abstract thinking, which is great in English class, but awful in terms of the paralyzing effects of self-consciousness.
In short, your son is reporting to you exactly how he feels, and you are reporting to him exactlywhat you see--both are true.