Sometimes a food allergy can cause both a physical rash as well as stomach/digestion problems. Does he have any known intolerance or food allergies? It's also possible they aren't related at all. Have you changed any laundry detergents, mattress, comforters, etc? I think you need to have him make an appointment with his doctor to sort this out. He may also need to do an allergy test to figure out the cause.
Also does anything help his rash? Benedryl, hydrocortizone, regular lotion, oatmeal baths?
Until you get to the doctor, try a exclusion method to figure out what may be causing it. Keep everything the same, but change one thing at a time. Change your laundry powder and soap, look for bed bugs and fleas. For food look at milk, soy, nut, and wheat products. The doctor will be interested in knowing things that don't work, to help figure out the cause.
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