My youngest started lying almost as soon as she could talk.By 2 she was an accomplished liar !! It is part of growing up ...You just have to consistently tell them it's wrong and make sure if they tell the truth they get in less trouble than if they lie. I have 2 other children and they were never convincing liars... but my youngest was. She lies less now. Just give a consistent message that it's wrong and hopefully he will do it less. Praise him when he tells the truth.