Maybe he was honouring him by allowing his father to have the sole opportunity to fulfill the mitzvah :-)

He’s not giving Avraham food or drink, nor helping him dress or walk. How would this be Kibbud Av?

@DonielF. As I said many times before, applying pre-Matan Tora people with S"H is a bad strategy, and even then, "honoring your parents" is beyoned the strict ruling - "ויש שהוא כודנו ברחיים ויורש גן עדן"

Maybe he was too young? Or maybe Avraham didn't let him because it was his job?

@Alaychem he was 37. I wouldn't call that young.

@Dani That's a Midrash which I aware of, but one can understand otherwise

@Alaychem The easy solution certainly is to assume that the Midrash which says that the Avos kept the Torah before is metaphorical, but if you take it literally, that they kept not only the written Torah but also the oral Torah and even Rabbinic decrees, then I see no reason why a Braisa defining what's included under Kibbud Av should be any less controversial than the premise of the question itself.

@Alaychem Even if you only use material in the Pesukim without Midrashim, you can't propose that Yitzchak was too young: after all, it takes a lot more effort to carry logs up the mountain (as v. 6 explicitly says he did) than it does to pile them up (as v. 9 explicitly says he didn't). That doesn't refute that Avraham wouldn't let him pile it up, but certainly it wasn't an issue of physical ability.