Is it permissible for a mother to ask her youngest son to wait until he marries so she can help him, as she did with his siblings, or must she write a plot of land in his name now?

The land left by the father is the property of the heirs and should have been divided. Since it was all sold, the brother must be compensated for his share in it. As for the mother's land, it is not inherited during her lifetime, and the mother is not obliged to give the youngest son like his siblings before his marriage. The brother's right from his father's land must be paid. If you are unable to do so, the mother can help by giving him a portion of her land in exchange for him relinquishing his right, as a form of exchange (mu'awadah) and not a gift (hibah (definition)).