What you do is explain that lying is not acceptable and that the consequences for it will make any other consequence had he just told the truth more severe...then do it. Every time you catch him in a lie punish him...ground him, take away his games, whatever. My children have been told from the beginning that the trouble they will get into for lying will be so much worse than if they are just honest with us. We tell them that their word is everything. Being honest is a very important trait to have. People don't want to associate with liars. As a result, my children would come home from preschool or elementary school and actually tell us if they had to change their color that day or if they had to be talked to. We would have no way of knowing were it not for them telling us because the teachers only write home when they go to red. My boys have never gotten to that but they've gotten to yellow before because of excessive talking for instance. For that, they don't get in trouble but we talk about it and how the behavior must change and I thank them for being honest with me. I think those little issues help with bigger issues and there have been a couple but I will say, my boys cannot lie very well. You might see them stumble for words because they are considering it but in the end, they tell us the truth and I know this because I check out what they tell me if it warrants it.
You know you have to get this under check now. What is going to happen when he is a teenager even at 14 or 15? You are not going to be able to trust him and those are the years where the level of trouble they could get into (drugs, girls, stealing, ditching school, driving unsafely, etc.) would scare me the most.
A word of advice, as hard as it may be, try to remain calm about it. Be stern but calm and follow through on your punishments. He has to earn your trust. Until then, I would be very involved in his life if you aren't already. What I mean by that is his room, backpack, checking up on friends' houses he is supposed to be at, etc. are all things I would consider fair game. Trust is earned. When he goes for a period of time without lying then he will begin earning yours back. Remember too that when he DOES tell you the truth, to thank him. I'm not saying to not still dish out a consequence if the situation calls for it but I would continue to be calm and thank him for being honest and maybe even tell him how his consequence would have been worse if he had lied. "Because of your choice you cannot play video games for 3 days. Just so you know, I appreciate your honesty and had you lied, you would have earned a week of no video games." Let him see that he got off 4 days because he was honest. You are still punishing the bad behavior but rewarding the honesty.
I wish you the best and hope this helps.
L.