Many possibilities.. ..Depending on his age, associated symptoms, and other factors. If it developed quickly it could be an allergic reaction to something that contacted his skin, bug bites, or irritation from underwear. Most of those kinds of rashes have external causes. Try 1% Hydrocortisone cream and if it persists, have a dermatologist look at it.
Possible allergy. This could be many things. Allergy to latex (which is in most elastic waist bands) is not unusual.