If his father asked him for money and he does not have any, does he have to take a loan?

Firstly: 

The father has the right to take from his son’s wealth whatever he needs, if that will not harm the son. That is because of the report narrated by Abu Dawood (3530) and Ahmad (6640) from the hadeeth of ‘Abdullah ibn ‘Amr, according to which a man said: O Messenger of Allaah, I have wealth and children, and my father is in need of my wealth. He said: “You and your wealth belong to your father.”

Classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

What the hadeeth means – as al-Khattaabi (may Allah have mercy on him) said – is that the father needed the money for his living expenses, but the son’s wealth was so little that his father’s living expenses would consume all that he had. But the Prophet (blessings and peace of Allah be upon him) did not excuse him and allow him not to spend on his father’s living expenses. See ‘Awn al-Ma‘bood. 

It is stipulated that the father should be in need of this wealth according to the majority of fuqaha’, apart from the Hanbalis, because of the report narrated by al-Haakim (2/284) and al-Bayhaqi (7/480) from ‘Aa’ishah (may Allah be pleased with her) who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Your children are a gift from Allah to you – ‘He bestows female (offspring) upon whom He wills, and bestows male (offspring) upon whom He wills’ [ash-Shoora 42:49] – so they and their wealth belong to you, if you need it.”

Classed as saheeh by al-Albaani in as-Silsilah as-Saheehah, 2564 

Ibn Qudaamah (may Allah have mercy on him) said: The father may take whatever he wants from his son’s wealth and take possession of it, whether the father needs what he takes or not, and whether the son is a child or an adult, subject to two conditions:

(i)that it is not unfair to the son, it does not harm him and he should not take anything that the son needs;

(ii)that he does not take from the wealth of one son and give it to another, because it is forbidden to single out one child for giving from his own wealth, so it is more appropriate that it should be forbidden to single out one child for giving what he has taken from the wealth of another child.

Abu Haneefah, Maalik and ash-Shaafa‘i said: He does not have the right to take from his son’s wealth except as much as he needs, because the Prophet (blessings and peace of Allah be upon him) said: “Your blood and your wealth are sacred to you, as sacred as this day of yours, in this month of yours.” (Agreed upon). And it was narrated that the Prophet (blessings and peace of Allah be upon him) said: “It is not permissible to take the wealth of a Muslim unless he gives it willingly.” Narrated by ad-Daaraqutni. And because the son has complete authority over his own wealth, so it is not permissible to take it from him.

End quote from al-Mughni, 5/395 

If the father needs the wealth, he may take from his son’s wealth and spend it on his own living expenses and those of his dependents, so long as that does not harm his son and he does not take anything that his son needs, such as his car that he drives, and so on. See also the answer to question no. 9594 

Secondly: 

If you do not have any money, you do not have to take out a loan, and you can tell your father: I do not have any money right now, or you can tell him that you need the money that you do have. This is not regarded as being disobedience towards him. 

It is not permissible to take a loan from a riba-based bank under any circumstances, because of the texts which indicate that the one who consumes riba and the one who pays it are cursed. And it is not permissible to obey your father if he tells you to take out this loan, because there is no obedience to any created being if it involves disobedience towards the Creator. 

To sum up: if the father needs the money to spend on maintenance or to buy furniture for the house and so on, it is permissible for him to take from his sons’ wealth, on condition that this does not harm them, and that he does not take things that they need. If they do not have any money to spare, then they do not have to take out a loan. 

The brothers should cooperate to help the one who has not got married, each according to what he is able to afford. 

And Allah knows best.