J.K.
You didn't mention your sons age. Is lying becoming a habit- like some kids say huh? all the time even if they hear you? My son had a lying habit- and that is what it was because he lied about stupid stuff that he wouldn't have gotten into trouble at all about. He was about 9. We are a christian family and so he had to look up verses about lying and copy them down. Eventually he memorized them. I also did the whole washing his mouth out with soap thing- then used hot sauce and other bitter things to remind him that a lying toungue will make life very uncomfortable for him. If I suspect him of lying I ask him if he is lying. He will usually tell me right off if he is. One time I could have swore that kid was lying but when I asked him and he said no, i accused him again and he got really upset about it and told me God knew he wasn't lying and there were tears in his eyes. I knew he was telling the truth so I apologized for not believing him in the first place- later his sisters backed up his story too. However it was hard for him to gain our trust after all the lying he had done.
I agree with Karen also that consistancy is necessary. Talk with him about what punishments he will recieve (like taking favorite things or activities away) for lying and how hard he will have to work to earn his things back which will help him learn how hard it is to earn trust back as well.
Pray for and with him. Let him hear you pray that God will help him to break this strong hold of lying. It is a hard one to break once you start.
You are also going to have to make sure you prove to him that he will get in less trouble for telling the truth then lying. For example- my son got a bad grade (we already knew about it) and he asked if he could have a friend over... we asked what grade he got. He told us the truth (that time) so we allowed the friend to come over but my son lost video games and tv for a week afterward. Once before he lied about a grade (after a friend was already over) so we sent the friend home, apologizing and explaining to the friend and parents why (which was embarrasing to my son) and he lost video games and tv for 2 weeks. He learned real quick that lying doesn't help.