Op, what is the school doing for your son in this situation? It sounds like he has a terrible situation. I'd report the girl to the school guidance counselor. HIPPA violations are very serious and she should know she is jeopardizing her mom's job by repeating private information. You could request that the counselor leave you out of it, and just say it was brought to her attention that she is speaking about another student's medical condition (maybe others overheard her and/or she has spoken to others about it) and how unethical and serious it is.
What did the school do about the assault? Does your son have a plan to keep this from happening to him again? Some school counselors have the victims of assault accompanied at all times in the school. It does help to thwart further assaults. Oh, and I would file assault charges. Maybe then the boy will think twice before sucker punching your son again.
Lastly, I would want to know. As others have stated, speculation is one thing. I would want it confirmed one way or the other. Leave it open ended, stick to the facts as you know them, and ask for an investigation. Also request a follow up with you on the matter. If the doctor didn't do it, great. If she did do it, it's out of your hands. Hospitals have protocols for this. The doctor of all people, knows it.
Good luck. It sounds like your son has really been through it.