Does a son have to spend on his father even if he is rich?

Firstly: 

The father is obliged to get his son married
if the father is rich and the son is poor and cannot afford to get married, because marriage is part of the obligatory spending. Allaah says
(interpretation of the meaning): 

“but the father of the child shall bear
the cost of the mother’s food and clothing on a reasonable basis”

[al-Baqarah 2:233]

So the father has to get his sons married if
he is able to and they cannot afford it. He has to spend on them with regard to food, drink, education etc, because spending is obligatory upon
the father. 

Secondly: 

The son does not have to spend on his father
unless two conditions are met: 

1 – The son is rich (i.e., he has what he
needs and more) 

2 – The father is poor. 

If these two conditions are met, then the son
is obliged to spend on his father. Here the questioner mentions that her father-in-law is a doctor and that his income is good. If that is the
case then the son is not obliged to spend on him. But if the father asks his son for something, even if he is rich, and the son does not need that
wealth and will not be affected by giving it to his father, then he should give it to his father lest he be regarded as being disobedient towards
his father. But if the son needs that money, he does not have to give it to his father; he should explain to his father the financial difficulty
that he is facing [?} and that after he had paid off his debts and has become financially stable he will send money to his father as much as he
can. 

“Allaah burdens not a person beyond his
scope”

[al-Baqarah 2:286 – interpretation of the
meaning]

And Allaah knows best.