This could be just an allergic rash. you can give him Benadryl and apply Calamine lotion on the rash .
If it subsides in 3-5 days then there is no need to get him evaluated further.It could be due to a lot of reasons, including food.
Allergies are relatively common. Both genetics and environmental factors play a role. In a person with allergies, the immune response is oversensitive. When it recognizes an allergen, it releases chemicals, such as histamines. This causes itching, swelling, mucus production, muscle spasms, hives, rashes, and other symptoms, which vary from person to person.
Watch out for a worsening in the rash, fever or difficulty in breathing, if present, take him to ER.