First, passengers who want a full refund are only eligible if they cancel their flight within 24 hours of booking or before the flight departs.
If it has been more than 24 hours, he can still cancel the flight but will be required to pay the cancellation fee.
Brussels Airlines Cancellation Policy says that Passengers may cancel their flight up to 4 hours before flight departure, subject to cancellation fees and only refundable airport taxes and other government taxes.
To cancel a flight or check the status of a refund, passengers can select the airline's "Cancellation and Refund Request" option available on the official website.
Passengers who have booked an award ticket and are about to cancel will receive a refund in the form of travel credits, which must be used within 90 days.