Hi Kingda61,
I'm sorry that this is happening to your son and I hope your son is recovering well.
Can you sue the school or the other child?
You will have an action against the child who caused the injury, or rather, their parents (assuming the child is under 18) for recovery of medical fees and other loss suffered by your son.
If the accident happened on school grounds and during school hours, you might additionally have an argument that the school (which owes parents, and children, a duty of care) was negligent or breached their standard of care by not taking adequate measures to minimise/stop the bullying, and in particular, to prevent the injury to your son.
What can you do now?