Rapidly losing sight?

Well.
have this kind of half-formed idea for Camp NaNo. I have a character in mind who is rapidly losing his eyesight, permanently.

I have no idea if this is even possible, or why this would happen. It would be ideal if he could become blind over maybe three to six months. This character would be about fifteen or sixteen years old, too. Is there some kind of condition that would cause this to happen? An injury?

Sorry if this is an insanely stupid question, but this is the one thing I'm having trouble researching because I'm not sure what I'm looking for.

Does he have to go completely blind, or can legal blindness suffice? You have a lot more options with legal blindness.

Retinal detachment, cataracts, glaucoma, tumour on the optic nerve, keratoconjunctivitis sicca (dry eye: http://en.wikipedia.org/wiki/Dry_eye)...