Grandsons do not inherit when sons are existent. (Part No. 16; Page No. 530) Therefore, you are not entitled to receive any share of inheritance of your mentioned grandfather because your paternal uncles exclude you from being heirs.
Bequeathing one third or less for non-heirs is permissible but the bequest becomes effective after the death of the testator. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.