Start with the Principal and the teacher, you can ask for a weekly report about your son's safety. If you feel that his safety is still at risk, then contact the school board and the PTO/PTA and share your concerns. If your son is being bullied on the playground, many schools have volunteer parents that are used to over see recess and lunch rooms. If the time is available to you, becoming a volunteer could allow you to see what is going on and may be reduce or eliminate bullying for your son. Many children are not very good at self advocating verbally, you can role play things to say with your son. Most schools also have a bullying program in place. Utilize that resource as well. Keep in mind that the school cannot legally tell you who the other student is or what consequences they have issued. They can only tell you that they are taking care of the situation and that the bully was consequated.