Hello S.,
have the doctor do a blood test to check for allergies... one thing, he'll have to be off the benadryl for a few days. talk w/ the doctor. I'd wait a few days before taking him off the benadryl to make sure his skin heals well first.
Next I'd take him off of all milk products. change wipes, diapers, bath gel,
no baby oil: To make mineral oil, gasoline and kerosene are removed from crude petroleum by heating, in a method call functional distillation. By using sulfuric acid, applying absorbents, and washing with solvents and alkalis, hydrocarbons and other chemicals are then removed.
http://www.bodymindbreath.com/content/BabyOil
skip baby powder... get the mildest and NATURAL of baby soaps....NO sodium laurel sulfates
skip the baby lotion.
figure out how you can let him "air out" from time to time.
besides the rash, is he 'growing' 'as scheduled' (what ever that means) what I'm asking, is he showing any signs of not absorbing nutrients?
also, ask the doctor if he's too young for yogurt. could he have candida/yeast? Good luck and keep looking! follow your gut. ~C.~