Will the father be rewarded if the son regularly recites from a Mushaf he inherited from him, even though reciting from this Mushaf is difficult due to its small size?

May Allah forgive your father and have mercy on him. The signs you mentioned indicate a good ending for him. Your recitation from the Qur'an that your father left behind will bring him reward, because the Messenger of Allah (peace and blessings be upon him) said: "Indeed, among what continues to reach a believer of his deeds and good deeds after his death... is a Mushaf (copy of the Qur'an) he bequeathed." So, your recitation from this Mushaf benefits your father and benefits you due to your righteousness towards him. Note that the Mushaf is part of the inheritance, and it is permissible for you to take it if you are the sole heir, or if the heirs relinquish it to you, or if it is counted from your share.