Wow, this is a tough situation. (how's that for stating the obvious?)
My family just moved here from S. Florida. My son started high school here as a senior and had a rough go of it in the beginning. Teenagers are so self centered and don't reach out to anyone new.
Is your son friends with this other boy? Have they known each other for a long time? Do you know his parents, even just a little?
I would contact the other child's parents. Give them a chance to discuss what has happened. If they are non-responsive, then I would take it to the school administration. Did this happen on school grounds? If so, the school is liable...especially since they did not provide your son any help at all.
If you feel very strongly about it, I would consult an attorney, not one of the ones who will sue at the drop of a hat, you know some of them see the school and a lawsuit as easy money, which as you know, it will not be. The burden of proof would be on you...school's these days, especially public ones, are so slow in their responses, they have to make sure they have not opened themselves up to being sued.
I understand you and your family are shaken and it is not right that you alone should bear the medical costs.
What was the fight really about? I would dig a lot more into the incident and try to speak to the other child's parents before you do anything else.
I hope this all turns out okay. Good luck. Feel free to write me back. This is our family's first encounter with public schools. The ones my children attend are all 'five stars', but we have not been that impressed.