I don't know who the doctor would report anything like drug use to unless he is on probation or something like that where he has to report it. As far as I know the doctor can't even tell you because of patient confidentiality since your son is not a minor.
I don't even know if that is something that would show up on the test unless it was an overall blood test. Drug tests, I believe are more directly related to finding certain drugs and may not even be involved in the testing he is having. Not sure if it would come up or not.
Heidu