Unfortunately, if Luke blocks you, you can't contact him again.
Don't you have any more recent saves before he blocked you completely?
If you're really far into the story of the other characters and you can't go back and start Luke's story again, what I can do is try to release a feature in a future release to reset Luke's story only - I already did this in an old release number 6.0...
But I can't guarantee when I'll be able to make this special version for you. First I need to deliver the next release and then, when there's time left, I'll make it for you.