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.
It's sometimes necessary to give a child more complete information about ADHD and why medication is necessary for him.
My ten-year-old is on medication for ADHD, and he's starting to resist taking his medication. My friend says let him be responsible for his own actions and let him take responsibility for his actions. I say no to that because he needs to understand that the medications really help him concentrate.
Have you tried calling Children and Adults with Attention Deficit Disorders (CHADD)? They may have a branch near you where you can get some support for your son. The toll-free number for CHADD is 1-800-233-4050. They also have a website at http://www.chadd.org. Branches throughout the country offer a range of services including regular parent support group meetings to meeting opportunities for children, adolescents, and adults with ADHD.