My ex-husband says he cannot afford child maintenance because he now has a new family. Is that a valid excuse?

A new family does not cancel his responsibility towards the children of his earlier marriage. Each of his children is entitled to maintenance (nafkah), and he must plan his finances taking all those responsibilities into account. The Prophet ﷺ said: “It is enough for a person to be sinful if he withholds food from those under his care” (Sahih Muslim, Kitab az-Zakah). The amount can indeed be adjusted if his income has genuinely changed, but that must go through honest discussion or a court decision. It cannot be done by unilaterally stopping payment. Keep a record of every payment received or in arrears. If there is already an order and he does not pay, the Family Support Division (Bahagian Sokongan Keluarga) at the Syariah Court can help with enforcement. Read the explanation in enforcing maintenance.