hi,thanks for query.Since it a drug allergy
,the allergic symptoms like hives
will come and go till about 48-72 hours till the drug is completely excreted from the body.this is common and there is no need to worry in the absence of any other complaints.Continue with benadryl
,it contains anti allergic drug which should help.This usually work,if not you may need to give some another anti allergic drugs,Please take a note of this drug reaction and avoid giving it next time.
wishing him good health.