Hoping for your complete recovery, OP 
I’m with your son on this one. Good for him. If the school has a zero-tolerance on violence policy, like my son’s school has, he’ll get the same punishment as the other boy, unfortunately. Definitely question the school about what is being done to address the vile child who started it.
Last school year my son was attacked by three boys during a class (one of those workshop classes where kids work in groups). The teacher didn’t see the other boys jump my son, but naturally he saw my son defending himself and push and punch one of the boys attacking him.
My son had never, ever been in trouble before, while one of the perpetrators was a constant troublemaker. All four of them got hauled down to the principal’s office, and because of the zero-tolerance policy, all four of them got a three-day in-school suspension. To sort out the mess, the administrators questioned some other kids in the class, who verified that the three bullies started it, completely upending the lies the bullies were spewing that my son began the whole thing.
My son overheard one of the principals saying (behind closed doors) to the other, “Well, what was Tilez Jr. supposed to do? He was jumped by three boys!” The assistant principal, when he called me, was almost apologetic on the phone, but had to adhere to the zero-tolerance policy, which I completely understood.
I don’t like violence either; however, I did have one comment and one question for my son: “Next time, it may not be such a good idea to retaliate as you did, although I see why you did it,” and “Did you land some good punches at least?” He said yes.
On the plus side, those same three boys have been working to get into my son’s good graces, and he’s now “cool” with them. He won’t be best friends, but since he’s got a bit of a rep now, they leave him alone, and no one else has tried anything with him since.
I imagine it will be the same with your son, and bullies will know to steer clear of him now.
Sorry for the long post. Bullies everywhere rile me up to no end.