Hello and welcome!
If you're talking about medications you'll need to know about side effects, what adverse reactions have been noted in clinical trials, what to do to prevent or offset side effects and how to proceed in the event of an adverse reaction. You need to understand dosing, what to do if he misses a dose, etc. Also, how to contact him/her after hours. in my opinion, you also want to discuss what types of therapy/behavior modification will be most effective for your son because there are studies that show that medications & therapy tend to give a better outcome than medications alone.
Good luck and update us on how the appointment goes!