Some Laws of Mourning When the Grandfather Passes Away and the Father is Not Alive

His father passed away a few years ago, and now his grandfather has passed away, and several uncles are sitting shiva for his grandfather. Does he have to observe some mourning laws until Shabbat?

He does not observe mourning customs at all, as the custom to observe mourning practices is out of respect for his father or mother who are sitting shiva, and since they have passed away, there is no reason to observe this mourning.

Written in Pitchei Teshuva, Yoreh De'ah, Siman 374, Se'if Katan 4: "See in Shut Adnei Paz, Siman 11, where it is written about someone whose relative, disqualified from testimony, died elsewhere, and he has mourners here who do not yet know about it, that he does not need to change his clothes according to the custom on the first Shabbat. Since the main custom is out of respect for the mourners. And it is also implied in the Rema, who wrote out of respect for the mourners with him, therefore here, where they are not mourning, it is obvious that he is exempt, and there is even a prohibition due to disrespect for Shabbat. And it seems to me that what the Rema wrote, and nevertheless it is customary, refers even to when there are no mourning relatives here, and it is also explained in the Darkei Moshe. Therefore, he needs to change his clothes. And as explained, the source of the custom today is to mourn only together with his father and mother, and specifically when mourning for their father and mother, therefore, it is certainly out of respect for his father and mother, and not out of respect for the deceased, and therefore, when they are not alive, he does not observe even partial mourning."