K.D.
Wow, I sympathize. That is a tough situation. Maybe next time he comes over to play outside with your kids you could have a 'meeting' and tell all the kids what the rules are for good play behavior and if the rules are broken that child will have to go back to their house. Your kids would have to go inside and the neighbor child would have to go back to their house. And also specify that we don't get to 'try again' until the next day. I would stay outside and supervise this play and then send him home when he acts up. Then tell your kids to let you know if he comes back so you can send him home again. He needs boundaries that are enforced and it sounds like he is not getting that at home. He will fall into line, I think, if you stick with it.
As to whether he is adhd, autistic, or odd - not sure. He could be odd and if the consistent boundary setting doesn't work, then that is a possibility. Even adhd kids can learn boundaries and be good playmates. They might be hyper and lack focus is all. As to autistic - does he look you in the eyes or your kids in the eyes when he talks. Does he actually talk conversationally with your kids at all? I would look up the symptoms of these diagnoses on the internet and see if you think he matches. Maybe if he does, you could leave a printout of the symptoms for that diagnosis in the parent's mailbox - or maybe you will have to anonymously mail it to them since the boy may notice.
As to that grandson, I would inform the neighbors of every incident and see if that helps.
Good luck!