I cannot help but notice the affection that I see here in this interplay. “My father!” “My son!” Does Isaac suspect anything? At this time have they begun to discuss what is going to happen? We do not know. Isaac is no dummy. He might have figured things out. This question here may have been seeking confirmation. No doubt he could not help but notice his father acting a little strangely; gets up early in the morning; chops wood; they travel for three days to make a sacrifice; but there is no animal.
Abraham is a type of God, Isaac a type of Christ. And as such, Isaac has complete faith in his father; just as Christ would have rather not gone through the pain of crucifixion, He did so, trusting His Father completely. I think Isaac was the same way.
He had hope. "Well, if God will have me kill him that will not stop God's promise. He could raise him from the dead. Isaac will live." There was hope for life. And so, he went through it.
And that answered Isaac's question. They were in agreement. They walked together, because not only did Abraham have this hope, Isaac too had this hope because Abraham was an excellent father. The two went up on the mount in agreement that God would provide a lamb for the offering. And God did, in the next few verses.
You can see now that Isaac had not been clued into what was going on. So they did not talk about the sacrifice on the way there.
