Hello. Thank you for writing.
Has he been in contact with anything unusual? A rash is not a specific diagnosis. Instead, it refers to any sort of inflammation and/or discoloration that distorts the skin's normal appearance. Common rashes include eczema, poison ivy, hives, (and athlete's foot). Infections that cause rashes may be fungal, bacterial, parasitic, or viral. There is no way we can know what this is.
If it worsens, see a doctor.