Like many pre teens, you son has skills which require quick responses such as games and the computer.  These tend to appeal to this age group due to quick screen changes and visual affect.  In order to provide a more successful work skill, be sure he is not hungry, rested, and can do his homework in a place without distraction e.g. no music or tv (including IPods).  Take frequent breaks and be sure the study surface is free of clutter.
 
At the age 7-9, many bright males show Attention Deficit Disorder Inattentive Type.  They are experts at things that show rapid change such as video games but do not have classic concentration to achieve homework as we have known it for a century.
 
Good luck but talk to his teacher to see if she/he has some suggestions also.
 
Wayne Yankus, MD, FAAP
expert panelist: pediatrics