All perfect praise be to Allah the Lord of the Worlds. May Allah`s peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
The general rule is that the father should treat his children fairly when it comes to gift giving and shouldn`t single out one of them without a lawful excuse. This is to preserve love and unity and prevent hatred between the children themselves and their father. This also keeps the father on the safe side before Almighty Allah since some schools of Islamic jurisprudence obligate gifting children equally.
The evidence on the prohibition of singling out one of the children in gift giving is reflected in the following narration: An-Nu'man b. Bashir told that his father brought him to God’s Messenger and said, "I have given this son of mine a slave.” He asked whether he had given all his children the same, and when he replied that he had not, he told him to take him back. A version reports him as asking, "Would you like them to show you equal filial piety?” When he replied that he would, he said, "Don’t do it then.” In a version he said that his father gave him a gift, but' Amra* daughter of Rawaha said, "I shall not be satisfied till you call God's Messenger as witness.” So he went to him and said, "I have given my son from ‘Amra daughter of Rawaha a gift and she has ordered me to call you as witness, Messenger of God.” He asked whether he had given the rest of his children the same, and when he replied that he had not, he said, "Fear God and act equally with your children.” He said that he then returned and took back his gift. A version quotes him as saying, "I shall not be a witness to oppression.” * She was the wife of Bashir b, Sad al-Ansari and mother of an-Nu'man (Isti'ab, p. 746). (Bukhari and Muslim.) Another narration supporting this prohibition is that Allah's Messenger (PBUH) said: "Be afraid of Allah, and be just to your children.' My father then returned and took back his gift." {Bukhari&Muslim}. However, scholars have disagreed on what is intended by the prohibition mentioned in these narrations; some said that it indicates that it is almost unlawful while others said that it is unlawful.
It is conditioned in Sharia that the father doesn`t give gifts to his children with the intention of depriving one of the heirs from his/her share of the inheritance. The evidence on this is that Allah's Messenger (PBUH) said: "Be afraid of Allah, and be just to your children.' My father then returned and took back his gift." {Bukhari&Muslim}. Intending to deprive the heirs from their lawful shares of the inheritance violates the command of Allah Who clarified the rules of inheritance and the share of each heir. Rather, some scholars gave the person in charge of inheritance affairs the authority to take back the gift if he realized that the intention of the giver was to deprive the heirs. Ibn Battal said: "Al-Mohallab said: "the person in charge of inheritance affairs has the right to take back the gift if he realized that the giver`s intention is depriving other heirs. This is since (but she ordered me to make you as a witness to it) indicates that she didn`t approve of singling out her son until the Prophet (PBUH) was a witness to that, and this proves that the giver`s intention was to deprive the other heirs. In some narrations, it is stated: ( I shall not be a witness to oppression.), and it was known that An-Nu'man b. Bashir favored the mother of that son. " {Shareh Sahih Al-Bukhari Vol.7:P.104}.
However, favoring a child with more gifts for a specific reason such that he/she enjoys a special virtue or has a special condition: poor, ill or needs money for tuition, and the like, then favoring him/her is permissible.
In addition, scholars excluded disobedient child concerning giving-gifts. This is attested to in the following narration: "Imam Malik reported that Aisha (May Allah be pleased with her) said that Abu Baker gifted her a certain amount of his money." {Al-Mowatta`, Vol.6:P.1089}. Ibn Hajar {May Allah be pleased with him} said: "It isn`t reprehensible to favor one of the children in gifting in the following cases: depriving an evil sinner to prevent his spending the money in sinful acts, giving more or favoring the most needy or distinguished child, as Abu Baker did with Aisha." {Tohfat Al-Mohtaj, Vol.6:P.308}.
In conclusion, it is disliked for a father to treat his children unjustly and without a valid excuse when it comes to gift-giving or intend to deprive some of the heirs from the inheritance. Therefore, if the disabled son, exclusively, can`t provide for himself and needs medical attention, it is allowed for the father to allocate a certain amount for his expenses or register part of his property in his name and he (father) is cleared from liability before Almighty Allah. However, we advise this father to explain this ruling to his other children to prevent sowing discord between them. And Allah the Almighty knows best.