What would you tell his Mother?

I am writing my post today on the case study that we didn't get to talk about in class on Tuesday September 30th. The case involved a concerned mother and a teenage boy who was showing symptoms of increased lethargy, dizziness, and confusion. The boy and his mother visited the doctor due to his mothers fear that the son had been bitten by mosquitos recently while being outside. The mothers concern is understandable due to the fact that at the time West Nile Virus, which is carried by mosquitos, was becoming a problem in the area. After extensive blood work and multiple diagnostic tests, all the results came back normal and more tests were going to be run to find out what was wrong. The doctor orders was preparing another round of tests which included a spinal tap. The doctor is asked by the boy to tell him of his condition only when the mother is out of the room. The boy confides to the doctor that the reason of his confusion is due to him and his friends over consumption of Dextramorphan (cough medicine). He tells the doctor that his mother cant find out about what is causing his condition. The mother is still convinced that her son has West Nile Virus and asks the doctors if any other tests can be done. What should the doctor do?
       This situation has to be quite common in the medical field and is most likely handled on a regular basis. As a doctor you are obliged to not tell the mother due to doctor patient confidentiality between the doctor and the boy. If I were the doctor, I would either do one of two things. I would try my best to convince the boy to be honest with his mother and tell her that he has been taking Dextramorphan with his friends. If the boy doesn't do this, the mother will most likely ask the doctor about the private conversation. The only response the doctor can really say is that the patient has informed me that the information discussed cannot be shared with others. If the doctor says this, he is basically saying that the boy the boy is hiding something from his mother that she doesn't need to know. In a way the doctor is violating patient confidentiality without actually violating it in the sense of the definition. The mother will know the boy is hiding something.The other option would be for the doctor to convince the mother that the spinal tap and more tests don't need to be done because nothing seems to be physically wrong with the boy, other than confusion and dizziness. The likelihood of this working is slim because the mother wants to find out what's wrong with her son and by any means will want to get her healthier, even if that includes a spinal tap. This most likely would not work, but it would uphold the doctor to patient confidentiality between the doctor and the boy. Either way, the doctor knows now of the boys condition and is obliged to keep their conversation between them. More extensive procedures are not needed at this point so the situation is now between the mother and son and not the doctor. In my opinion the best way is for the boy to come clean to his mother and accept the punishment that will most likely follow. The boys mother will find out one way or another.  What would you do in this situation? Would you tell the boys mother? What would you do if you were in the boys shoes?