S.S.
When my son was two he had something that sounded similar. The hives just showed up one day and came and went for days. We had been using the same detergent and no new foods either. So after a number of days we took him to the pediatrician. She put him on Zyrtec for for 10 days and it never came back. We never figured out what the hives were from (we were to come back for some testing if they had returned). I personally like Zyrtec over Benadryl because it is once a day dosage so you don't have to remember when you last gave it and also it is non drowsy (for my kids anyway).
Hope he is better soon.