All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Since your grandfather offered this piece of land to his grandson while he was a sane with all his senses functioning properly, then it became the property of the grandson and it will be added to his inheritance and will be divided among his heirs. His brothers (brothers of the grandson) will not inherit him because he had a son (who will inherit him).
Allaah knows best.