Halakhah for Kiddushin 147:5

And the father is believed [when] he says, "This is my son, my first-born" (Kiddushin 74a), or to say, "This is my son" or "This is not my son." As it is written (Deuteronomy 21:17), "But the first-born etc. shall he recognize" - and they, may their memory be blessed, expounded (Bava Batra 127b), "shall make him recognized by others." And even if someone was presumed to be Reuven's son, if Reuven says that he is not his son, he is believed, and he does not bequeath to him. And Rambam (Mishneh Torah, Laws of Inheritances 4:2), may his memory be blessed, wrote, "It seems to me that even if the son had sons, [such that] even though [the father] is not believed about him to say he is not his son, for the purposes of family relationships, and he is not presumed to be a mamzer on his word, he is believed for the purposes of inheritance and he will not inherit him.