I'm sure it's very frightening for you as a mother to see your child hurt in such a serious way. It's important for you to get the entire story from your son about what happened and document it thoroughly.
You will have to speak to an attorney if you want to take legal action against the perpetrators. However, there are other steps you can take to deal with it before you get to that point.
The first step is to talk with the school administration about the incident. Make an appointment with the principal when you and your son can sit down and speak to him/her about what happened. Consider taking a written copy of your son's description of what happened including any other times in the past that he's been injured due to bullying.
By making the school aware of what happened and asking that action be taken by the school, you are setting a precident that bullying will not be tolerated.
I'm so sorry that this has happened to your son. I guess "by law" you always have the right to call the police to file an assault report.
But there's a lot you can do before that point to help your son resolve this issue. The first thing to do is talk to him to try to get a really clear understanding of what happened. You're not trying to figure out who's "fault" this is, just the story of all the steps that led to the point where your son was injured.
Next, make an appointment to talk to your son's teacher. Share the whole story with her - and don't leave out any parts where your son may have some responsibility for the situation. Of course no one ever deserves to be bullied or injured, but there may be some history of conflict between these kids that your son played a role in. Make sure you leave that meeting with a clear understanding of how the teacher plans to resolve this situation and what ideas she has for how to protect your child in the future.
If you're not satisfied with her response, make an appointment with the principal. If that doesn't go well, speak with someone at the district level. Don't give up until you know that the situation is being handled appropriately. Every school has to deal with bullying these days. If you're school isn't doing it well yet, you'll be doing all the kids and parents a great service but insisting that they get better at it.
Education.com has a great special edition about bullying. I'd encourage you to check it out - especially the articles on what you can do as a parent to help a child who has been bullied.