Must a young man give money to his father who does not pray, and who spends money without management, while there are other siblings who work and give him money, and the young man is saving for marriage and building a home, and giving him money regularly will delay his marriage for many years?

Filial piety and kindness to one's parents are among the most obligatory duties. And among the acts of kindness to them is spending on them if they are poor, which is a consensus among scholars. A condition for the obligation of spending is that the child must have what exceeds their essential needs, and they should start with themselves.

If you fear falling into what is unlawful (haram (definition)), then saving for marriage expenses takes precedence over spending on your father. However, if you do not fear falling into what is unlawful, then you must spend on your poor father in a reasonable manner, and maintenance is obligatory upon all of his affluent children.

As for if your father is not in need of money, the majority of scholars believe that it is not obligatory for you to give him. The Hanbalis hold that a father may take from his child's wealth whatever he wishes, even if he is not in need, provided that he does not cause hardship to the child's wealth and does not take to give to another child.

You must be dutiful and kind to your father in all circumstances. And if your father and siblings do not pray, then among the greatest acts of piety towards them is to command them to pray and to warn them against abandoning it.