Thanks for the advice.
Your messages came a little late, but what I did in the end was very similar to what you suggested. I ended up cancelling my son's ticket because they were asking for HKD1500 for changing to April because the ticket had to be used within 3 months of original departure.
My son wouldn't be able to leave then so I cancelled it and was offered a refund for the taxes since the ticket was strictly non-refundable.
I guess it's better than nothing. Thanks again!