It's essential to ask your child why they don't want to get married again. Here, it's must knowing the social & mental attitude towards their second marriage.
Whether their spouse has passed away or they're divorced, it's crucial to make them realize they're going through aloneness and it's better to have a companion.
Stress, anxiety, and fear can grip the person. If they're already a parent, then it could be tiresome for them to care the child.
Less stress, depression, fear. Social & emotive strength from the spouse. Better healthy behavior & a greater sense of purpose achieved through a spouse.
Taking care of your child is your priority after the death of their spouse or divorce. It's needed you to explain to them that spending life on memories isn't good.
You must consult a psychiatrist to get your child out of depression. Psychiatrist will give your child a new direction to think and consider second marriage.
This will make your child aware of the personal and social values of being a life partner, which will give the child an eagerness to get married.
Find a suitable life partner of your choice for the child's marriage or if they have liked someone then talk to their family about the marriage of both of them.