Since his teacher did not offer suggestions at the conference, bring your son's very serious problems to the principal's attention. Request that your child be studied as soon as possible by a team of educators to determine why he is refusing to do any schoolwork and how he can be helped. Some testing as well as classroom observation will be necessary.
Beyond having the school start working on your son's problems, think about how things are going for him at home. Has something radically changed at home? Look hard and carefully to see if you can pinpoint any problems. Things that affect parents can seriously affect their children.
Have you tried talking to your son? Was he at the parent/teacher conference? Has anyone even asked him why he does not do classwork, answer questions on tests, or turn in homework? Is he a behavior problem in school? Does he get up every morning and go to school willingly? There are so many different questions that you need answered. A school counselor or psychologist may be able to help both you and your son get him on the right path.
Don't worry about grades, homework, and tests now. Concentrate on getting to the root of your son's problems, and the other things will come around.