Make sure that your son knows that your family values honesty and that you expect him to always tell the truth. Tell him that just as with the boy who cried wolf, once people know that someone tells lies they don't trust him or her anymore. This means that when you suspect that he may not be telling the truth, you will check to find out.
Set up a positive reward system so that when you learn he has told the truth, he can earn computer time or a walk around the block just with you; when he has not told the truth he does not get computer time or a walk. The key to any system like this is to give the rewards consistently and over a long period of time.