First, please try and loop in to your meetings the School Counselor and School Psychologist. They may be able to provide to you insight to help your son.
Also, children who are bright sometimes need motivation that is external to the classroom situation to help them complete work. Such as a behavior chart which marks off how many days he completed his work with reasonable expectations. After a few days he can receive a special privilege, such as staying up an hour later on the weekends or going to a favorite park with friends and family. Like adults, children sometimes need to be rewarded for their efforts.
Lastly, try and bring some multimedia in to his work. So, if he asked to do math problems perhaps he will enjoy doing them with fun pens or different colored markers. Maybe he will like to use hands on activities to augment his assignment, such as cooking to learn about fractions.
Here are some websites that may be helpful in addition to what you have available on Education.com
first i would pay close attition to his or her everyday life. Try to figure out if it is daily chores or just book issues. Have him or her tested through and outside oppinion then ask the school of theirs. You should b able to get a better understanding.