child allergies

my 2 and a half year old son is allergic to penicillin. his pediatrician prescribed Omnicef for severe ear infections. She said there was a 5% chance hed be allergic to that but she wanted to try it out. well, he was allergic. After 3 doses he broke out in HIVES all over his face and scalp scratching and screaming in a way no child ought to scream.
I immediately gave him benadryl and stopped the meds (wed at 9am was the last dose) it is now friday afternoon
he has no fever but is still breaking out in some hives. they go away on their own.
he hasnt had benadryl in over 15 hours because he hasnt seemed umcomfortable. i did put cortisone cream on him when he was starting to scratch but i am only using it 3 times a day.
my question is how long will this medicine take to leave his system and how long will these outbreaks keep occurring
?it is now Friday and he hasnt taken the medication in 47 hours. the breakouts are NOT severe at all and he is not complaining.

[Sandra Y, RPh]

Omnicef is eliminated from the body within 9 hours, and therefore should be eliminated completely by now. The hives should stop soon.

[tinapastina]

why do the hives keep coming long after 9 hours?