Try to show the expense you involved in your son, and then ask for special items on top of HKD 5,000. Say travel, buying computer, course for learning.....in a excel and request ex. HB to pay, if not paying, asking society worker for helping....
you can express the purpose of loan is due to ex HB late payment and/or insufficient payment, the court will consider your ability of earning money rather than load status (unless your loan is quite large, and it will affect your living standard for family, if your load is for your son, it will not cause any adverse effect.
Mediation is a way to resolve your dispute with your ex HB