Hello, My Grandson 15 Years Old, Complained Of Hives, Itching,

Hello, my grandson 15 years old, complained of hives, itching, throat tightening after going to park; he came back home it was worsening, Mom gain him Benedryl 2 tabs. EMS arrived administered O2....BP 132/64,P 60. Earlier 1 hour before park he ate shrimp what possibly could have caused this reaction.......He has eaten shrimp before and other shellfish.