I'm sorry to here your son is not wanting to go to school. There must still be something going on. It is important to keep the lines of communication open between you and your son. Here are a few things you might want to try in order to figure out exactly what is going on
Ask your son specific questions like:
Who did you eat lunch with today?
Did you see the kids who were picking on you?
Did anyone say something to you that you didnt like?
Are those kids picking on anyone else these days?
How do those kids treat you now?
What do the teachers do when they see someone being picked on?
Talk to other parents. You don't have to go into details, but ask them what they see or hear from their kids. Sometimes they can give you additional information you never heard before.
Set up any appointment for a parent/teacher conference. Share your concerns with your sons teacher. Maybe he/she can shed some light on what's going on.
Volunteer in the classroom. This canbe one of the best ways to see what's happening in the classroom. Look to see how others treat your son and how your so treats other people. You might be able then to give better advice at how you can help or give advice to your son on how to handle certain situation.