My girls doesnt have allergies, but my old pup did and we went trough a lot of trial and error with him...i found it normally took between 2-3 weeks to really make a difference with his food, but he was quick to repsond once we found the right one! Also, i noticed that with him it was not about one particular food to avoid, really think it had more to do with the preservatives that are used.
Hope you're little guy is alright!! i know it's difficult. Benadryl works welll, so does an oatmeal bath and for sore spots i used diaper rash cream (zinc cream) on him...it stopped him from scratching!!