The idea a father can forego his honor is not obvious on its own since the Torah writes "Honor your father and your mother". It comes from a gemara in Kiddushin 32a
Rav Yitzḥak bar Sheila says that Rav Mattana says that Rav Ḥisda says:
With regard to a father who forgoes his honor, his honor is forgone,
and his son does not transgress if he does not treat him in the proper
manner. By contrast, with regard to a rabbi who forgoes his honor, his
honor is not forgone.
The Rambam in Mamrim 6:8 writes similarly
Although these commands [of honoring parents] have been issued, a person is forbidden to lay
a heavy yoke on his sons and be particular about their honoring him to
the point that he presents an obstacle to them. Instead, he should
forgo his honor and ignore any affronts. For if a father desires to
forgo his honor, he may.
Despite looking for them, I didn't find other statements on the details of this law, or other statements to that effect.