I actually just a couple weeks ago had a 16 week aracana who one day was fine, and the next day i went and checked and she looked like your youngster does...the itself was swollen closed, gunky, and a knot behind the eye is where i think the swelling originally come from, see i thought at first it was a sting as well, till another one had the exact same problem pop up a few days later. Well i had pulled the first out immediatly from the other 3, then when the 2nd happened, they ended up being together in the shed in 'quarantine' and i made a quick simple run, on the other side of the house from the main chicken coop. These guys come from my hens i've had for a couple years, and never had any problem with any of the others.
Longer story a bit shorter, the first one, i treated it with the eye ointment, washed the eye and area out with saline, and a small dose on penicillin every day....4th day after it started, the swelling had gone down, but whatever it was deteriorated the eye and it litterally fell out of the socket - yes very gross, had a bit of smell, upped the penicillin to a 'normal' dose, by day 7, the area was healing up nicely, swelling going down, ect.
Chicken number two, was seen with swollen eye on day 2 after first one, so i upped this ones penicillin the same day i did the other one, and i had put electrolytes in their water, and #2 Chubbs, swelling went down, the area is bare, and her eye is 'squinty' and i dont know if she has sight in it, though i think she has some because she is vigilant with either side.
Now, two weeks later, both chickens seem fine, and almost fully healed up, but are still in isolation from everyone. But someone else has suggested that my flock has MG, and that my older hens are carriers not infected, and the young chicks ended up getting it from their mother hen. Since my hens had gone broody, i had let the silkies and one aracana/silkie cross sit on eggs of a few of each of my breeds' eggs, and i let the silkies raise the babies, so they were continuously with the group of hens in the smaller 'mama coop' .....
Now i dont know if thats what they have, and i still dont think so since they nor any other chicken in my flock is showing any other signs of MG, even so I have had them tested and am now waiting on results, I don't know exactly what I am planning on doing, I know i have seen other posts from other BYC'ers have had MG invade their flock from new birds that were introduced without being tested and not showing signs...But i got all of my elder hens from Ideal and McMurray back in '06 and '07, and the others are these guys' offspring...But yes i know i could have infected them myself....
I suggest you test this sick bird, only this one at the moment, and see if anything is up. I'll feel better knownig one way or the other, and when i get my results back, i will make my decision on which course of action after the fact.
Good luck with this guy and any decision/action you take.