If you want to solve the issue and protect your son, find the best solution to all parties. To me, calling police doesn't make much sense if your son didn't feel hurt in any way.
Sometimes, "time out" may do more harm to a child than a "slap". Kids want to know they are being loved, cared and included. A teacher told me isolating a child from group is the harshest punishment without inflicting any "visible" injuries. Similarly, cold war in a family is not good to kids for the same reason.
A book I read explained why kids need certain discipline. It's about security. With discipline, kids know a “boundary”. They feel safe and secure within the boundary and know they will get “reminded” if they cross the boundary.
