Hi,
This could be an allergic reaction to some unknown substance or dermatitis or eczema or insect bites.
He needs to wash the areas several times with fresh water. He should not use any cosmetic products at the sites and use a medicated sunscreen whenever you go out.
Apply calamine lotion at the site of the lesions and see if it helps. He could take some oral antihistamine medications like cetrizine or loratadine.
It would be advisable to consult a skin specialist for his symptoms and a proper clinical examination.
Let us know if you need any other information and post us on how he is doing.
Regards.
You can develop rash when youre' allergic to just about anything. If you are certain it's a food allergy, you need to determine what food it is and avoid it. Meanwhile, have him take some Benadryl (it can make him sleepy though).