Zakaah is the greatest of the pillars of Islam after the Shahaadatayn and prayer, hence the one who withholds it is committing a grave sin which is one of the major sins.
Because zakaah is an act of worship, it is not valid unless it is done with the proper intention on the part of the one who does it, because the Prophet (peace and blessings of Allaah be upon him) said: “Actions are but by intentions.” So he has to pay it himself, or give it to someone to pay it on his behalf.
If a man gives money in charity but he does not intend that to be the zakaah of his wealth, then he wants to make it the zakaah of his wealth, that is not valid, because he did not have the intention of paying zakaah at the time when he gave the money.
Similarly, if someone wants to give zakaah on behalf of someone else, that is not valid unless the owner of the money gave him permission to do that, because this permission shows that he has the intention of giving zakaah.
Ibn Qudaamah said in al-Mughni (4/88):
The view of most of the fuqaha’ is that the intention is a condition of giving zakaah, because the Prophet (peace and blessings of Allaah be upon him) said: “Actions are but by intentions.” End quote.
Based on this, your taking the money from your father and giving it as zakaah on his behalf without his permission will not mean that his duty has been discharged, and it will not lift the burden of his sin for withholding zakaah. You should carry on advising your father.
We ask Allaah to guide him.
And Allaah knows best.