There are many things that may be causing this behaviour- I would suggest you get him evaluated by a professional psychologist.
Possibly depression, ADHD or may be caused by the death of a loved one or a sudden change of circumstances e.g. moving house, parents divorcing or remarrying or new family circumstances. Could be being bullied at school, have a critical teacher or have a learning disorder.
As mentioned it would be best if you could get him evaluated by a professional in order to get to the bottom of this. Perhaps he is simply being a 'normal' child and we are placing too much pressure on him- to be sure let him speak to a professional.