Should he give his zakaah to his son because his salary is low?

It is not permissible for a father to give his zakaah to his
son, because if the son is poor and the father is rich, the father is
obliged to spend on his son, so if he gives him his zakaah, it is as if he
is giving it to himself. 

Ibn Qudaamah (may Allaah have mercy on him) said in
al-Mughni (2/269): The obligatory charity (zakaah) should not be given
to parents or to children. Ibn al-Mundhir said: The scholars are unanimously
agreed that it is not permissible to give zakaah to one’s parents in a
situation where the giver is required to spend on them, because giving
zakaah to them means that they no longer need him to spend on them and that
he is spared that duty, so he benefits from it, and it is as if he is giving
it to himself. 

Similarly, he should not give it to his son. Imam Ahmad said:
Zakaah should not be given to one’s parents or child, or son’s child, or
grandfather or grandmother, or daughter’s child. End quote. 

There are two exceptions: 

1 – If the son is in debt, in which case it is permissible to
give him zakaah, because the father is not obliged to pay off his son’s
debt. 

2 – If the father does not have enough to spend on his son. 

Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him)
said, in al-Ikhtiyaarat (p. 104): It is permissible to give zakaah to
parents and other ascendants – meaning grandfathers and grandmothers – and
to children and other descendents – meaning grandchildren – if they are poor
and he is unable to spend on their maintenance, and also if they are in debt
or are slaves who have a contract of manumission with their owners, or are
wayfarers. If the mother is poor and has small children who have wealth, and
spending on her will affect them, then she may be given some of their
zakaah. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was
asked: Is it permissible for a person to give zakaah to his son? 

He replied: That depends. If he wants to give it to him when
he is obliged to spend on him, then that is not permitted. If he wants to
pay off a debt on his behalf, such as if there has been a car accident and
the car that he hit was damaged, and the car is valued at tens of thousands,
then it is permissible for the father to pay it with his zakaah because of
this accident. End quote from Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen
(18/508). See also (18/415). 

And Allaah knows best.