With our daughter (a second grader), we've noticed she does better with completing her homework when we sit near her, guide her through the assignments, and answer any questions she has. Her teacher also provides specific tasks that require parent involvement. Before sending completed homework back to the school, she and I review the work she has done, and I help her with making corrections to errors (explaining what was wrong and why).
Talk with your son's teacher to see if he or she has any specific tips for the situation with your son. You might also consider rewards and consequences related to homework.
Sometimes our schedules are so hectic we forget that our children are LEARNING something we already know. Someway we expect them to know the subjects rather than seeking our help..We feel they are utilizing our brains rather than they using their own. How about we set some time aside to join in and NOT give answers but guide them to a path of least resistance so they can just about figure out how to get the results themselves?????