oooh this sounds heaps like what happened to T last year ... he had toncilitis
and was then put on pennicillin for it over the next few days he developed a splochy rash on his face which would appear strongly like he had been slapped then dissapear then 10 mins later on the other side of his face ... when i took him to the doctor OF COURSE it had gone completely and my doctor explained they hadnt had an outbreak of slap cheek in tassie for a long time

anyway just as we were leaving the doctor saw the rash appearing on him and said "oh thats an allergic reaction is he eating new foods?" i couldnt think what it was till i got home and immediately stopped the pennicillin .. he has never had it since

i hope your little man is feeling better