If Annie is correct, then I would strongly suggest looking into the laws and helping your parents go what they need to do to put him on notice ASAP. Brushing this under the rug won't make the problem go away, and it sounds like he's a ticking time bomb. That's not good.
Best to you, hope to hear back from you!
In most states, if you move in with other people like in this situation, you do have tenancy even if you never paid rent. An eviction notice has to be served with 30 days notice.
He cannot refuse to move out, well he can, but that's not his decision. Unless he has a tenancy agreement and is paying rent, his grandparents can kick him out at any time, for any reason, and if he's abusive, I would recommend that happening ASAP. If he refuses to leave or gives them a hard time, they can call the police, or petition the courts.
Obviously your son isn't living with you, why? Did he cause trouble for you before moving in with them? Are these your parents? How old is he? Does he have a job? It would be super if you could give us a little more information.
I'm sorry you're going through this.
As you are posting on the depression site, do we assume your son has been diagnosed with depression? Do you think his behaviour is due to this?
Sounds like a very tough situation. Can other relatives maybe step in too?
Maybe have an intervention of sorts and get him evaluated. His anger issues could be helped in therapy.