It sounds like your son is at the point where he needs to be given more information about what ADHD is and why this medication is necessary for him. An excellent book for kids his age is Putting on the Brakes: Young People's Guide to Understanding Attention Deficit Hyperactivity Disorder by Patricia O. Quinn and Judith Stern. It has a very nice section about understanding medication that could be helpful to your son.
Son Is Resisting His ADHD Medication
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