Praise be to Allah.
We put this question to Shaykh Muhammad ibn Saalih al-‘Uthaymeen, may Allaah preserve him, who answered as follows:
He has to give his father what he needs to get married, especially if his father is in great need of getting married.
Question: if the father wanted to build a house and asked his son for the money, should he give priority to his own house or his father’s house?
Answer: he should give priority to his own house over his father’s house.
And Allaah knows best.