A son who is very old, or a great scholar, or leader [i.e., Nassi or king]:
Even if a son is a great Torah scholar, and even if he is very old, he still remains obligated in the Mitzvah of honoring his parents. Even if he is a leader such as a Nassi or king he remains obligated in the mitzvah. Even a son who is the main Rebbe of his father in Torah learning is obligated to honor his parents although with some limitations as explained in chapter 5. Nonetheless, some Poskim rule that a son is exempt from performing those matters of service for his parents which are beneath his dignity due to his old age or scholarliness, or prestigious position.
 Encyclopedia Talmudit Erech Kibud Av Vaeim Vol. 26 p. 425
 Meiri Kiddushin 31b; Darkei Moshe Y.D. 240:7
 See Kesubos 103a and Haflah there; Melachim 1 2:19 that King Solomon bowed to his mother in her honor; Meiah Shearim 36
 Ramban Toras Adam Inyan Keriah; Ritva Bava Metzia 33a in name of Ramban; Gilyon Tosafus Shita Mekubetzes Bava Metzia 32a