Could it be that he is in the very early stages of puberty? I have heard that children are developing earlier these days. Sounds like teenage problems / attitude - he's twleve right? Maybe he's confused about some things that he just simply feels too shy to speak with you about. Perhaps he will talk to another male member of the family? What about other problems at school? Could it be that he is being bullied? Has he had a new teacher lately (my son was very negative about school in the beginning as he didn't like his teacher, now, he is getting better grades because he can relate more to his new teacher).
I hope you find the answer and that things improve for you both.
Hang in there