Why did Yitzchak mention that they have the fire and wood, but not the knife?
Abarvenel: Because it is normal to take a knife to fend off bandits or Chayos.
Apiryon: He knew that the knife is for Shechitah. If it is for a regular Korban, we should have taken a Seh. If Hashem commanded you to slaughter me, fire will descend from Shamayim to consume me. There is no need for fire and wood!
Meshech Chochmah: The knife does not show intent for a Korban; it could be for Chulin. Fire and wood for an Olah must be new (no person used them. If so, we should have taken a Seh!)
Chasam Sofer (Chulin 15a): Avraham took a sharp wooden knife. 1 It was included in the question about the wood.
Divrei Yatziv YD (4:10): According to Rav, we can say that it was covered, so he would not need to check it again before Shechitah. Yitzchak did not see it.
Tiferes Shlomo: He could understand that Hashem commanded Avraham to slaughter him, to return the Neshamah to Shamayim. However, it is not proper to burn human flesh and bones!
Why does the Torah say "Aviv" and "Avi"?
Bereishis Rabah 56:4: Yitzchak appealed to Avraham's sense of mercy [not to sacrifice him] after the Satan told him "the ornaments that Sarah made for you will fall to Yishmael!" 1
Ohr Yechezkel Midos p.40: Before this, Yitzchak was ready to be Moser Nefesh. How did this dissuade him? The Midah of jealousy appears even in Gedolim!