My child has had a low grade fever and some cold symptoms since monday(mild cough).today he developed hives as well.what can i do aside from benadryl (diphenhydramine)?

Hydration & perhaps: a longer acting antihistamine like zyrtec or claritin (loratadine) (depending on your childs age). It sound like your child had a viral upper respiratory illness, & hives are often associated with viral illnesses, along with many other possible causes (food, medications, environmental exposures, etc). Hives often get worse the 1st few days, then get gradually better. If wheezing, swelling develop see Dr ASAP.