This is a very important topic to discuss. I'm glad you raise it up.
Bullying is a very common problem everywhere and all the time, it can happen anywhere, in school, in home with their classmates and even in your own home when you are on the site. It just happened to my son during the march break. I invited a friend's son to come over to my house to stay a few days. I have to say his son is not a behaved boy, but he is lonely and need help. I took two days vacation to stay at home to provide more activities to them. But even with parents in the house, my son was still been bullied and suffering. Last night he told me the boy insulted him all the time. I actually knew that already but didn't think it was that serious. But after I heard what my son told me what happened, I realized I have to teach my son some skills of anti-bully.
First of all, I told my son that he is a very smart, nice, responsible and reasonable boy everybody knows, especially loved by mom and dad. I told him I knew he sacrificed a lot to invite the bad boy come over. He took over his bed and room.
Then, I encourage him to think a few solutions to protect himself. For example, I asked him how he should do to be able to protect himself and he gave a few solutions and I discussed effectiveness of each one.
I used to provide suggestions to him if he encoutered the same thing. It is supposed to be more effective than suggetion if your son can think out a way to handle it. Of course, if your son is too young, I think you have to help and encourage him. I believe this is an important skills everybody should learn no matter what society you are in. I still remembered I was bullied a lot when I was young. Unfortunately, there are always not enough help around. I think parents should be educated in terms of how to provide help to their kids. I suggest you read more books and think more, and discuss more. As long as you not giving up too quick, you will find your effective way to protect your kids. I'm also learning, but I believe there are no easy way to this kind of problem. And also you have to mostly count on yourself. Let's encourage each other.