All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If the court has ruled that your son is obliged to pay that amount of money, then there is nothing wrong with you paying the zakaat of your money to your son if he is unable to pay it off due to his debt, because the debtor is one of the zakaat recipients. And the fact that he is your son does not prevent you from giving your zakaat to him as it is not obligatory on the rich father to pay off his son's debt. So, it is permissible for the father to give the zakat of his money to him so he could pay off his debt.
Allah knows best.