Hi C.!
One of my biggest problems when my oldest girls were young, is that I had a real problem with consistency. My girls didn't listen, because they never knew what to expect, and I usually never followed through with what I said. What I would do is to explain to him that not telling the truth is bad, because then people never know if they can trust what he says, and that telling the truth is always best, because otherwise he's just making a bad situation worse. I would set some rules as to what he can expect if he lies, or misbehaves, and stick to it every time, like time out. Let him think about it in time out for 5-10 minutes, or until he wants to tell you the truth. He'll respect you, and feel more secure if he knows what the consequences are for his actions.