Make sure that his study area at home is free from music, TV, and excess noise. He should have the supplies he needs and good lighting. His study time should be consistent -- for instance, after dinner. Don't have him do homework right after school; let him play and relax first. One and a half to two hours each weeknight is a reasonable amount of study time. Encourage him to work on the most difficult subject first and save his favorite until the end. It is perfectly fine for you to be there from time to time to ask how things are going, review spelling words, or help practice for a test. If your son doesn't have homework, require him to spend the study time reading and/or reviewing material from his classes.
Most students do try to get good grades. Sometimes they just don't know how to do it. With your help and the cooperation of the school, I am sure your son will learn how to be successful.