All perfect praise be to Allah, The Lord of The Worlds; and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
In principle, if the donee had taken possession of the gift, it is impermissible for the donor/giver to take it back, as indicated by the Prophet`s Hadith which reads, "One who gets back the gift is like one who eats vomit"{Bukhari}.
However, the Shafite jurists made an exception in the case of what a father gives to his son and grandson, as stated by Al-Imam An-Nawawi(May All bless him), "If a father had offered a gift to his son or grandson, then he is allowed to take it back. However, brothers, paternal uncles and other blood relatives aren`t allowed to take their gifts back."{Shareh An-nawawi Ala Muslim(11/64)}.
Moreover, jurists have conditioned that the gift should be in the possession of the donee/son in order for his father to be able to take it back, as stated by Al-Imam An-nawawi(May Allah bless him). Refer to {Minhajj At-talibeen(1/171)}.
In conclusion, it is permissible for a father to take back the gift or the waiver which he had offered to his sons. And Allah knows best.