We can appreciate your concern, frustration, and likely exhaustion regarding your situation with your grandson. Reaching out to us is a brave and wise thing to do, yet we know it is not always easy.
It wasn't mentioned if he has ever had a mental health evaluation. If not, this might be a good place to start and could give you some insight as to the type of intervention that could be most effective. It also sounds like much of your parenting energy is being absorbed by your grandson; that can often create stress with the other children and is not fair to them.
At this point, it is important to understand he is not likely going to change on his own, or with the strategies you have used to this point. That is not a judgement; it sounds like you are trying to be proactive, but he is presenting you with extremely challenging behaviors. If you continue to do the same things the same way, things will remain the same with his misbehaviors. So, how can you develop a new approach? There are parenting classes that can help and many good books on the subject, including “Common Sense Parenting" which is based on the model Boys Town uses with kids like your grandson. The Boys Town parenting website, www.parenting.org, offers guidelines, tips, articles, and information about establishing a new approach. On the home page of the website, you will see a link, “Boys Town's Parenting Principles" and another on “Parenting Guides," where you can find this information.
The Boys Town National Hotline is another resource that can offer assistance and support, as well as referrals that can be helpful during this time of adjustment. They can be reached 24/7 at 1-800-448-3000.
Stay brave, keep reaching out.