I would just say something like "wow" or "neat" and change the subject. I have a lot of practice because my brother is that way with everything he talks about. Everyone knows he's talking out of his ass, but if you want the subject to end, just ignore the guy and pretend that you're listening.
It's kinda like being on a date with an annoying woman. You aren't paying attention to her, but if you want her to think you are listening so you can get some, so you pretend that you are listening and that you care.
