D.W.
The brat diet is good for diarreha. Bananas,apples,rice and toast. May be the milk is giving him diarreha? Try switching to lactose free milk or soy milk and see what happens. put him in the bathtub with some baking soda for the diaper rash. when he is done let him get some air to his butt before you put the diaper back on. sometimes they can also get diarreha or loose bowels when they are teething. if he is in pain give him some hylands teething tablets to help him sleep. sorry he is not feeling well. good luck.