Is there any way to allow location services again for an app (or at least show the dialog) if the user previously chose the option Never?
You need to manually allow the permission on that specific app to reset the request permission dialog.
Open the App info (either by dragging from app launcher, or Settings - Apps - [app name])
Select Permissions
Enable the permission you denied with "never ask again"
(Optional) Disable it again from here; note that by this time, your app will request the permission again when needed.
Link: https://android.stackexchange.com/questions/125939/reset-the-request-permission-dialog-on-marshmallow