Snarlatron makes some good points. Common courtesy goes a long way.
I think you can advise your nephew not to express whichever views you think may offend people, but it won’t change the fact that those are his beliefs. The root problem isn’t what he’s saying, it’s what he’s thinking that leads him to say those things. I don’t think you’re going to be able to change his belief system in 2 days. The best you can probably do is repeat what Snarlatron said and hope it sinks in.