Son did his interview when arriving at MDW. Was quick and painless.
"Once a traveler is conditionally-approved, instead of scheduling an interview at a Global Entry Enrollment Center, the traveler can complete the enrollment interview during CBP primary inspection at a participating airport. Upon arrival, travelers will be directed to a primary booth designated specifically for Enrollment on Arrival. A CBP officer will conduct both the primary processing and Global Entry interview and collect the traveler’s biometrics to complete the enrollment. The traveler will then be cleared for entry into the United States and will be a Global Entry member."