Hi! I believe I can be of help with this issue.
First, let me say I can imagine how overwhelming this situation must be for you. On the one hand you are trying to be loving grandparents. But on the other hand you see more and more clearly that your grandson is just using you.
And this is actually the key to my answer to you that you need to consider and think about. I am concerned that you are treating the lying and stealing as if they are the SOURCE of the problem. They are not! They are the RESULT of the problem.
If that's the case, then what is the source? The source is that he is USING people and manipulating people for his needs, wants, impulses, etc. His parents have clearly given up and are letting him be a parasite on you because they are tired of it. You will soon be tired of it and he will seek some other people to use.
And this is the cycle that has to be broken. But, you can't break it by ENABLING him to use you. So, you have to set firm boundaries. This is the key to everything. He can ONLY receive any help from you if he adheres to the boundaries. No chances. Any breaking of the boundaries and he forfeits rights agreed upon.
So, you need to decide if he is going to be allowed to live in your home to begin with. I think you should give him a three week eviction notice. But that is up to you. You can offer him financial assistance IF he meets certain guidelines.The two of you need to decide what they are. I recommend you find a psychologist for him and come up with the guidelines with the psychologist. He would have to for example, go once a week to sessions (maybe useful, probably not if he is not interested). If he lies,financial help withdrawn. If he steals, all help withdrawn. You see the pattern here.
The idea is that he has to learn how normal human interactions work: consequences, etc. And the family has to make sure that it (you) aren't complicit in keeping him thinking that he can get through life without learning them. He may be sociopathic and never learn these rules of normal human interactions. I don't know. That will be determined by what happens when he has to finally actually be dependent on himself. But he needs that opportunity.
So, please do this for his sake. I wish you the very best!
Please remember to click the green accept button. Feel free to continue the discussion; my goal is to get you the best answers possible. Bonuses are always appreciated! If I can be of further help with any issue, just put "for Dr. Mark" in the front of your new question, and I'll be the one to answer it. All the best, XXXXX XXXXX