8 year old nephew is starting to lie

My nephew has started lying and I don't know why he also is acting like he's a 13 year old and he is 8.we have tried everything he gets better then he will do it again a few weeks later it usually happens at school.he argues and talks back at home its disrespectful and very frustrating because growing up my sister and I would never it wouldn't fly.any pointers would be great.nothing rude please.we love him so much he is a great kid he is just not getting it.