A few questions....
1-Who diagnosed your son. Often the diagnoses of ADHD is done as an easy way out. There are neurological tests which sould be done prior to any medication.
2-have you looked at other resorces, often ( in mild cases) you can alter diet, exercise level, and even work on handwritting skills to improve the symptoms.
3-medication is not all the same, you can get herbal and homeopathic medications, you can get Rx medications, I have even heard of a case where the treatment is an antihystamine.
As for telling him, I would suggest that you just let him be part of the conversation. Talk to your pediatrician and let him be in the room, ask his imput, let him ask some questions. the more involved he is in the treatment process the more likely he is to follow it. (BTW Ritlan kills hoseplants, and kids who dont wantthere meds DO manage to spit it out no matter how carful you think you are) His co-operation is going to be a must, so get him onboard as early as you can so he feels that it was His decision and that he Wants to.