If one’s parents ask him to do something that is not beneficial to the parents in any way whatsoever (though not harmful to the parents, the child, or anyone else, either), is he nonetheless obligated to fulfill the request?

ב"ה Shalom Please pardon me, but I feel I can only answer this question theoretically but not on the practical level, since there are many variables, which I feel only a prominent Rabbi who knows all the details should give a practical ruling . Rav Yisroel Dovid Harfenes shlit"a addresses this question at length in his sefer "Vayivarech David", on the laws of "kibud Av Ve'em" (Siman 85,pg 236) . It is summed up in a bulletin "Halachically Speaking "in English of the Kaf-K Kashrut (Volume 11, Issue 30) pg. 12 . Conflicts With Kibud Av V’eim, https://thehalacha.com/wp-content/uploads/Vol11Issue3.pdf What I will say in short is that in general a child or teenager living with his or her parents and is dependent on them, should obey the wishes of the parents. However, if the parents make a strange request such as wearing a sweater on a warm day, there are those who say that there is no Torah mitzva for such a request, and it is up to one's will if the person wants to do so. If he decides to do so ,he will receive reward. Some say it's a Rabbinically enforced mitzvah. Some say it is part of the mitzva of "mora" of parents =showing reverence, while others hold that reverence only refers to issues which may show disrespect, not to things which have to do with one's personal issues. Whle others again say, that if not listening to them it will cause the parents pain they must be listened to. So, again I sum and say that all written here, is theoretical and should not be quoted as a practical ruling. All the best