You can also add the line GRUB_SAVEDEFAULT=true into the file /etc/default/grub then run grub2-mkconfig -o /boot/grub2/grub.cfg and it will add save_default=true into the /boot/grub2/grubenv file for you.
Once that is done then the last kernel booted will remain the default until the user selects a different one for booting on the grub menu.