I think getting directly involved could make matters even worse for your child. Going to the bully's parents, you risk the parent (potentially a bully themselves) being offended by your remarks but also the kid may get in trouble and retaliate on your child.
I feel the best resolution is to find an adult (teacher presumably) that you can speak with discretely. Don't ask them to act on the past but be vigilant going forward. This will draw their attention to the situation and perhaps then the boy can be caught redhanded and lessen the chance of revenge.
Ultimately, no one can be over them all day and it may continue. Reassure him/her that there are jerks in the world and that he/she must not take it to heart. IF the bullying is physical, I tell my children that IF, and only IF, left with NO choice, you defend yourself. I don't believe my child should take a beating to avoid suspension due to zero tolerance. (Sorry so long)