Lie to him a few times.

We're going out for pizza.

Then just drive around and come back and have soup

Yeah. I lied. Think about it.

At 15 years old, he knows what is truth and what is not. Call him out.

Always call him out on the lies. ALL of them. It's a character flaw that will only get worse with time.
Just look at 45.
I'm not joking.
I've been going through this with the 5 and 10 year old. I call them out on
every single one. I explain to them that there is no good reason to lie to me
about anything. Especially since I always catch them.
I explain about trust and honesty being important to every relationship they're
ever going to have throughout their entire lives.
I also explain the consequences they will face every time they lie to me.
There are ALWAYS consequences.

Probably a negative self image is driving this, he needs attention but doesn't think he deserves it. Don't pretend with him, be clear that you don't accept his account because you will accept him as he is. He is trying to appear more likable and interesting than he feels inside. This was me at 15.

Call him out. Every single time.

If he doesn't get called out, his lying habit will stick, and it will carry on into adulthood, in which case it won't just ruin the lives of those around him, but it will also ruin his own life too.

I have someone in my life who is a compulsive liar. I feel such rage over the relationship, that I don't even acknowledge it properly. But I truly believe this person is damaged in some way, and they can't help it. That's how I don't explode.

The biggest lesson I learned about lying was listening to the story of Peter And The Wolf when I was a child.

No one likes a liar.

He's probably just seeking attention. I would engage and communicate with him. Approach it slowly but directly. I suspect that the more you talk to him the less reason he will have to lie. He may only be seeking communication, children have a way of acting out when they feel neglected and that the adults in their life are not listening to them. Remember, communication is key. Good luck.

He has a bright future in a certain regime...

First, does he know he is lying? Is this something his parents are aware of? There could be any number of reasons why. Is it attention seeking? If that's the case any response other than ignoring can reinforce the behavior.

Maybe the story about the boy who cried wolf would be appropriate.

If it becomes chronic or serious it could be a sign of a mental health issues. For example: Distortions of reality, revising facts to fit feelings, and prone to visions/hallucinations. Personality disorders, for example often manifest in adolescents.

I have a niece who lied as a kid and now does it as an adult. Call him out on it. Liars are disliked by their school friends employers and even families can just get sick of it all. This habit/tendency will not get better and could lead to a very troubling adult life. A lot of liars also steal. They are accustomed to not being found out. If you don’t call out the behavior, you rewarding it. That’s just my view. Hope it might help.

If it is tall tales I would encourage him to record them in some way.

I believe people lie for one or more of 3 reasons.

1. They are ashamed of the truth.
2. They are afraid of what the consequences of the truth will bring.
3. They don't feel they are interesting or lovable enough without the embellishment of lies.

I think for your great nephew it's the 3rd. This is a tough one, because the shame (from calling him out) can reinforce any feelings of inadequacy. However, lies that are not confronted can turn into behaviours that hold serious consequences in the future. Age 15 is too old to dismiss as childhood fantasy.

Naturally you want to help and you want to do the right thing, but I think this is beyond your ability. I think the conversation you need to have is with his parents. He's young enough that they still have control and can take him to a psychologist or seek family counseling. It's strange to lie about something, like the example you gave. It's important to understand why.

While he's with you, you can chuckle when he lies and say "Now we both know THAT"S not true." Then change the subject so it doesn't gain momentum and teaches him his lies are transparent.

Declare your house a falsehood-free zone, fine him every time you catch him in a lie. Nothing teaches a lesson like the one that hits the wallet.

Nip it in the bud.
Nip it, nip it, nip it.
Tell him lies are not the sign of a strong character, they are the habit of a weasel.
People of integrity subconsciously group others according to their actions, and this will haunt him in the long run.

Leave him alone. Calling him out will alienate him, and he'll distance himself from you. Don't try to change someone's behavior unless it's more serious or harmful, or unless you're a behavior expert.

I think it's important for him to get honest feedback--from somebody. If everyone allows him to think they believe his tall tales he may not get the information he needs to build a trustworthy reputation/image. And you may be in the unique position to provide such feedback in a wholesome (=maybe without condemnation or embarrassment) way. Best wishes!

Question him on the lies and question him on the obvious truth. Show him that if he can't be trusted to tell the truth all the time then he can't be trusted ever.

Call him on it every time...use humor if you can, but he is trying out that "I'm 15 I know everything" Crapola and needs the wake-up call!

If you don't call him out on it, he will continue to think it's ok to continue lying and it will turn into bigger and bigger lies as he grows up.

Call him out on the issue in a calm, but firm way. Set limits with consequences and point out to him that the decision is his and that any consequences he will be bringing on himself.

Try to treat him like a friend. Be intimate with him so that he feels safe and kind to love him and love him.

Some kids crave more attention than others.... A simple criteria could be "is he hurting anyone?" No? Move on. Other than that, it's easy to come up with tons of not very useful comments since we don't really know all the circumstances

Only advice is to reinforce reality/truthbut the delivery of the truth I feel is the question.

How is he getting rich he doesn't even take a paycheck it's given back to government to put back into the system do you even know what your talking about there's only been two president I n history that refused a paycheck Obama never refused a paycheck that ass was playing golf thinking he's Tiger Woods every weak on the people's money like Royalty waisting your money trying to make a name for himself with the Obama Don't Care network!

People have already called him out, as sure as the sun rises and sets. But he still lies. Why?

don't steal and don't lie: everything else kind of falls into place. in the same situation, I was advised to refer to the liar as a 'truth extender'. he incidentally was an adult. no excuses, tough love at 15, before it gets any worse and gets him into trouble you can't get out of, because your memory of past lies trips you up. some say I am harsh, but the military does not deal with matters in any other way, and I believe they turn out good human beings, loyal, stoic, polite, punctual, tough and many other good personality traits.