Bullying is increasingly occurring in our schools and causing bad behaviors in our children. It's a tough situation to deal with because it's something that happens when most adults are not around. Probably the most important thing to your son right now is that you believe him and support him.
Talk to your son about specific situations that are occurring. After he tells you what happened, role play with him on how he can better handle the situation in the future. Have him practice telling you assertively to leave him alone or even to walk away. Practice with him on a daily basis so that these skills become comfortable to him. Once he becomes better with planned role play, try spontaneously having a practice session. For example, maybe you bump into him in the kitchen and tell him to get out of your way. He needs to stop and assertively and appropriately stand up for himself. Then when he does, praise him.
Work with your son on friendship skills. Teach him what are healthy and appropriate friendship skills and which ones aren't. This will help him determine that he can have friends and that not every child is a bully. This might mean increasing play dates at your home, so he gets more opportunities in front of you to practice his new skills.
