The words beginning with "But you say" (verse 5) have a quotation within a quotation. If necessary you can translate them as indirect quotations. "But you teach that a person does not need to honor his parents by giving them something that may help them if the person tells his parents that he has already given it as a gift to God" (See: [[rc://en/ta/man/translate/figs-quotesinquotes]] and [[rc://en/ta/man/translate/figs-quotations]])
It is implied that "his father" means "his parents." This means the religious leaders taught that a person does not need to show respect to his parents by taking care of them. (See: [[rc://en/ta/man/translate/figs-explicit]])
Here "the word of God" refers specifically to God's commands. Alternate translation: "you have treated the word of God as if it were invalid" or "you have ignored God's commands"