The Shulhan 'Aruch (240:18) writes that a Mamzer has an obligation to honor his father.
Why might I think otherwise?
Mi Yodeya is a question and answer site for those who base their lives on Jewish law and tradition and anyone interested in learning more. It only takes a minute to sign up.Sign up to join this community
The Be'er HaGolah there (#36) cites the source of this ruling as the Mishna on Yevamot 22a. The commentaries (eg Kesef Mishnah) to the Rambam's citation of this law (Mamrim 6:11) make this point as well.
The Mishna there says any son a man has, even a Mamzer (see how the Gemara derives this on the next page), counts as a son for laws like Chalitza/Yibbum, cursing, hitting etc. unless that son is from a non-Jewess.
So it seems the Shulchan Aruch and Rambam are just following the Mishna in this regard. As for why the Mishna had to say it, see the Gemara there which asks why the Mishna had to specify that a Mamzer counts as a brother for purposes of Yibbum. It answers that you might have thought the obligations only apply to Kosher relatives not Pasul ones. Perhaps a similar logic applies in the second case of the Mamzer son.