i'm not even sure rays can have allergic reactions, but i'd think it's possible. if that's the case, remember, the others would be able to eat the same things as him without the allergic reaction, just like 1 human could eat peanuts fine and the other would be hospitalized. don't rule out a food type just because the other rays aren't affected.
allergy was my first thought when i saw the swelling, similar to ppl who get stung by bees, eat shellfish, etc.
when did you first notice the swelling? it would definitely give you a better time frame to look at. was it around the time of the smelt? if swelling was before the prazi, it might just have worsened it, but shouldn't be declared the cause.