Praise be to Allah.
If the son needs to get married, and he cannot afford to do so, then his father must help him to get married if he is able to, because helping one’s child to get married is part of spending on him.
Al-Midraawi said in al-Insaaf (9/204): A man is obliged to uphold the chastity of those on whom he is obliged to spend [by helping them to get married], such as parents and grandparents, children and grandchildren, and others on whom he is obliged to spend.
This is the correct view according to the madhhab. End quote.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
A person’s need to get marriage is essential, and in some cases it may be like his need for food and drink. Hence the scholars said that the one who is obliged to spend on a person must also help him to get married, if he can afford to do that. So the father must help his son to get married, if the son needs to get married and does not have the means to get married himself.
But I have heard that some fathers forget how they were when they were young, so when the son asks his father to help him get married, he tells him: Get married by the sweat of your brow [i.e., earn the money yourself to get married]. This is not permissible, and it is haraam if the father can afford to help his son to get married; his son will hold him accountable on the Day of Resurrection, if he did not help him to get married when he could afford to do so.
End quote from Majmoo‘ Fataawa Ibn ‘Uthaymeen (18/410); Fataawa Arkaan al-Islam, p. 440-441.
If the father is not well off, but the mother can afford it, then she must help her son to get married.
Does she have the right to ask the father to repay what she spent, or not? There is a difference of opinion among the fuqaha’.
Ibn Qudaamah (may Allah have mercy on him) said: The mother may have to spend, and she must spend on her son, if he does not have a father. This is the view of Abu Haneefah and ash-Shaafa‘i.
If the father is going through financial difficulty, then maintenance is due from the mother and she does not have the right to ask the father to pay it back when his situation improves.
Abu Yoosuf and Muhammad said: She does have the right to ask him to pay it back.
Our view is that if it becomes obligatory for someone to spend on another person because of ties of kinship, he cannot ask for the money to be paid back, such as the father.
End quote from al-Mughni (8/212).
And Allah knows best.