Ever heard of the Hippocratic Oath? Or Doctor-Patient confidentiality? He's your doctor, he's going to probably give you lecture, but he would not, nor would it even be legally admissible, for him to nark you out.

Don't worry about it. He's a doctor, you could tell him how you murdered someone. The only thing he can ever report to the police is abuse (if you're a minor) and/or if you tell him you're PLANNING on killing/harming someone (or yourself).