If a child (or anyone for that matter) say's they want to kill themselves, you must absolutely react right away. He MUST be evaluated by a Dr. Quite frankly, for me, this would be an immediate trip to the Emergency Room.
Children do suffer from depression sometimes. This can be chemical and require medications. Other times it could be circumstantial - perhaps he is being bullied or something else is going on. Whatever the case, never ever ignore talk of suicide. Your child needs help. I wish you all the best, and hope that you are able to get to the bottom of what is happening with your son. Take good care.
I would find out if hes happy at school and how he gets on with other kids ..could be some bullying going on , has he any siblings sometimes there can be upsetting problems there .Does he watch TV or on the computer stuff that maybe scary ..Very often they say they don't know why when they are afraid to tell you ..Is his Dad around and how does he get on with him ?