What Is The Recurrent Rash Causing Breathing Trouble In A Child?

my grandson is 5yrs old for the last 2 yrs he has had a rash appear also with this rash his oxegen levels drop right down and he has to be in hospital for a couple of days on his last rash attack his whole face swelled up no one seems to know what is causing it he has had a lot of allergy tests done but nothing