Also, are you using hypoallergenic soaps and lotions on the rash? Vitamin E oil may be a good option also, since it promotes healing of the skin. Double-check with his doctor to make sure it is safe...
Something else is that the laundry soap, dryer sheets, etc., could also lead to skin rashes, so make sure to get the "hypoallergenic" or "free and clear" versions of laundry products if you want to cut down on perfumes and other known irritants.
I learned about food & other allergies the hard way (experience), and my husband has tons of food/other allergies too. Our house is free of the top 8 (wheat/gluten, soy, egg, dairy, tree nuts, peanuts, fish, shellfish) plus some...
Gluten & Sulfite Sensitivity, Multiple Food & Inhalant & Medication Allergies, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD (Lupus?), Osteoporosis, Anemia, Very Low Lymphocyte Counts (T-Cells & B-Cells), malabsorption/malnutrition, etc.
Meds: Pulmicort, Injectable Vitamin B12, Herbs, Nutritional Supplements, Essential Oils, Homeopathy.