Q. Can someone who has living parents recite Kaddish Derabbanan? Can one recite kaddish for a grandparent when the father is unable to recite, and the mother is alive?

A. Poskim (Even Shoham 20, Kol Bo Al Avelus 2: p. 145, et al.), write that one who has living parents can recite kaddish derabanan when there is no one else to say it. Horav Shlomo Miller’s opinion is that it is better if he consults his parents.
Poskim maintain that one can say kaddish for a grandparent with the approval of the parents.
Horav Shlomo Miller’s Shlit’a opinion is similar.

Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit’a