4yo nephew automatically lies about almost everything. How do i convince him to stop?

My nephew, 4m, hasn’t had the best time of it. His mom is an addict who is trying to stay clean and is constantly in and out of his life and constantly moving into new people houses, one boyfriend to the next and so on. His dad is great but never home due to work. They are divorced and has one older daughter, 10f, from a previous marriage.So while his dad, who is still working though custody issues with the ex, is working, I keep the nephew with my family. We have a 3yo boy who has become his best friend over the last 3 years of us keeping him Monday-Friday.His mom moved back and he started seeing more of her, we cannot deny her as she technically still has full custody now, despite still going though treatments. A few months ago the nephew started lying. He is caught in the act and he lies about it, blaming everything in anything else. My son doesn’t generally lie and although the two have been basically raised together in my home, the personality differences are immense and I’m not sure how to go about this new territory.My son, 3, is attached. He will do anything to keep from disappointing us. He is generally honest, toeing the line but usually admitting to any major issue. He knows the rule of the house, he lives here 24/7 so it’s easy for him to know what’s ok and what’s not.My nephew is a good kid. Is is kind and cuddly and loves spending time with my son and with us. We love having him here. Recently he has started lying. I don’t want to point at around the time his mom came back into the area but I honestly don’t remember it being so bad before that. I’m honestly afraid to send him to his mothers place but that’s another issue. My main issue is the first thought is a lie with him. I catch him getting on the counters, he blames the dog for scaring him. The dog is in the kennel and asleep, so that’s a pretty obvious lie. I hear they boys playing, I hear the pretty obvious sound of someone getting slapped and my done starts crying. The nephew says that my sone fell into the wall. The hand print is on my sons face, I know what happened and it wasn’t a wall.What in the world is going on? I’m afraid of what has been going on in the other homes he is in, that maybe this is more than just an age problem and something more sinister.I may never learn what caused this issue but maybe you all have an idea of how to combat it. I just need some tools to help keep the harmony in my home. via /r/Parenting https://ift.tt/3hpam8N