The sefer שער בת רבים brings from the Alshich and the Abarbanel that the reason that Yitzchok only asked about the wood and the fire was because he was asking a very clever question which would force Avraham to tell him the truth:
He asked: Behold, you brought fire and wood from home even though these are things which you can get anywhere, even in the wilderness - wood from one of the bushes, and fire by hitting two stones together - but where is the lamb for a burnt offering? You can't find this in the wilderness, so why did you not bring it from home?
But he did not ask about the knife because that is something which is not found in the wilderness and therefore it was understandable why he brought it with him from home.
Therefore, Avraham had to answer: "Hashem will provide for Himself the lamb", miraculously, and if not "for a burnt offering, my son" - you will be an unblemished offering to Hashem. And from this answer Yitzchok knew that Hashem had chosen him to be a burnt offering, yet nevertheless, "and they both went together" happily and with a complete heart - this one to slaughter and this one to be slaughtered.