5-year-old son lies through his teeth often. usually to cover up for some kind of misbehavior. is this age appropriate?

Yes: This is common behavior, and it's important to address it with your child. No need to be harsh, but be consistent and help him understand the problems with lying in ways he can understand. He only sees the benefits at this point.

He is normal : A 5 yr does not understand the importance of telling the truth. They often doesn't know the difference between reality and fantasy. He mimics what he sees. You need to teach him the importance of telling the truth, and encourage him to do this.