This may not be the case with your son, but I was also diagnosed with ADHD and ODD at a very young age - 3rd grade. I believe now that this was a misdiagnosis. I believe, my problem is actually a very mild form of Aspergers (which studies are find is VERY common). My sister herself was recently diagnosed with it though she was also originally diagnosed with ADHD at a young age. The two conditions are unfortunately very easy to confuse and between the two, a doctor is more likely to say ADHD because it is the more familiar condition.
As for how to help him, though I was originally on Ritalin and then Adderall, neither ended up being worth the side effects they caused. They made me feel empty and my teachers felt like they took away my personality, so I went off them. After that I continued to struggle in school despite consistently being one of the best writers in my grade. Like your son, intelligence wasn't my problem. I have 120 IQ. The biggest problem was that my standards made work take longer for me to complete and I'd become overwhelmed. In the end, what really ended up helping me wasn't a pill, it was learning to break down tasks into smaller ones, estimate how long they would take, prioritize and schedule accordingly. If you can help your son this way and instruct your teacher to do the same - breaking down bigger tasks into smaller ones, I think you'll be helping him tremendously!
Symptoms of asperger's are below. If this sounds like your son, I'd try taking the test at the link below and bring the results to your doctor:
test - http://www.wired.com/wired/archive/9.12/aqtest.html
Not pick up on social cues and may lack inborn social skills, such as being able to read others' body language, start or maintain a conversation, and take turns talking.
Dislike any changes in routines.
Appear to lack empathy.
Be unable to recognize subtle differences in speech tone, pitch, and accent that alter the meaning of others’ speech. So your child may not understand a joke or may take a sarcastic comment literally. And his or her speech may be flat and hard to understand because it lacks tone, pitch, and accent.
Have a formal style of speaking that is advanced for his or her age. For example, the child may use the word "beckon" instead of "call" or the word "return" instead of "come back."
Avoid eye contact or stare at others.
Have unusual facial expressions or postures.
Be preoccupied with only one or few interests, which he or she may be very knowledgeable about. Many children with Asperger's syndrome are overly interested in parts of a whole or in unusual activities, such as designing houses, drawing highly detailed scenes, or studying astronomy. They may show an unusual interest in certain topics such as snakes, names of stars, or dinosaurs.
Talk a lot, usually about a favorite subject. One-sided conversations are common. Internal thoughts are often verbalized.
Have delayed motor development. Your child may be late in learning to use a fork or spoon, ride a bike, or catch a ball. He or she may have an awkward walk. Handwriting is often poor.
Have heightened sensitivity and become overstimulated by loud noises, lights, or strong tastes or textures. For more information about these symptoms, see sensory integration dysfunction.