go immediately to your neighbor--parent to parent and explain what happened to your son. Be clear that you will not tolerate violence such as assault on your son and that if it continues you will report it to the police if you have not already done so.
If you have had good relations with your neighbors, have a two family meeting including their son and yours. Do your best to have your son understand that he cannot play around the older boys and should go back inside if they are out or go to the present adult so he can continue to play. Also, the older neighbor boy needs to understand his actions could wind him up in juvenile court. Both sides need to get to the root of the problem soon. Many states have anti bullying laws that apply outside of school as well.
Don't let this one drop.
Wayne Yankus, MD, FAAP
expert panelist: pediatrics