I agree with most of the posts and how Shelby summarized it.
Homeschooling is not a solution, only a band-aid. The real world will come one day. My Grandma protected her youngest daughter from the world and she's never been able to cope so still lives at home- she's turning 39 this year.
Our 4 year old son dealt with a bully and we did as the others say, talk to the teacher, always follow up and if you don't see changes there might be an issue with the teacher. In our case it turns out that the trouble child had been in her home daycare and now her pre-school since he was a baby so she was hesitant to talk to his parents or take much action. We found this out from another teacher who knew there was a problem with the bully.
Now our son goes to a private school and is a little sensitive to this type of situation. We talked to the school master just to give them a heads up and we also asked for advice. She said one of the most important things we can do is to not make too big of a deal out of it in front of him, let him know it's important for him to tell you but then take it to the next level and don't build up the drama in front of him. It seemed to work a little better.
Also one other thing we did was to make sure he had a circle of friends around him to help him know he is likeable. We did this by going to gymnastics, introducing ourselves to other parents of kids his age, signing up for mom's groups, all to get him around other kids. It seems to really be helping. Sometimes it just takes one good friend to make it all better, it's just sometimes tough to find that good connection.
Good luck, I know this is so hard to go through, we're still kind of dealing with it and each day is a challenge.