What Causes Red Dry Rashes On Thighs Of A Toddler?

my 3 year old son has rash only his thighs ,he previously suffered from milk allergies but this rash seems somewhat different. I gave him bath and during bath it gets reder and more irritated and normally it looks like a dry-ish red large spot all over his thighs...his allergy rashes would be all over his body and would go away after a bath and benadryl this hasn't really change