Let's first look at the context of this statement.
Avraham takes fire, wood and a knife and then proceeds with Yitzhok to the place where the sacrifice is set to take place. What is conspicuously absent is an animal to sacrifice.
Yitzhok, it seems, picks up on this and seeks to ask his father about this. But it also seems (from a careful reading of the pesukim) that Yitzhok is hesitant to bring up the issue (as he senses what the answer might be).
He starts, but doesn't say anything: וַיֹּ֨אמֶר יִצְחָ֜ק אֶל־אַבְרָהָ֤ם אָבִיו֙
He then tries again:
At this point he at least actually says something, but there is no question yet - just addressing his father. Avraham reply is inviting -- in essence letting his son that his free to ask:
וַיֹּ֖אמֶר הִנֶּ֣נִּֽי בְנִ֑י
It is at this point that Yitzhok gets the courage to ask the question: we have the fire and the wood, but where is the offering?
וַיֹּ֗אמֶר הִנֵּ֤ה הָאֵשׁ֙ וְהָ֣עֵצִ֔ים וְאַיֵּ֥ה הַשֶּׂ֖ה לְעֹלָֽה
And yet, he does not ask the question directly -- he is either leaving the question open-ended or he is asking it in an indirect manner. Either he thinks that appearance may be deceiving and perhaps he is not to be the sacrifice after-all or he thinks it inappropriate to ask such a question directly of his father.
My personal sense is that is both. Perhaps there is an explanation -- and if not, how can I directly ask my father if he is planning to sacrifice me.
So too, Avraham's answer -- is both open-ended and indirect. G-d will chose for himself the sacrifice (the word יראה is sometimes used in the Chumash in the sense of to choose). That sacrifice may be you, may not. And just like Yitzhok did not directly ask his father whether he planned to sacrifice him, Avaraham also did not directly tell Yitzhok that he may be commanded to sacrifice him.
In other words, Avraham's answer was a) relating to the substance and style of Yitzhok's question as well as b) relating to the fact that he did not know what was the Divine plan.