Why the sudden change?

Think of it as front stage back stage behavior. An example would be that waitresses and waiters act really nice and friendly in front of their customers of course, but when they go into the kitchen, their behavior is more likely different, like anger, cussing that sort of thing that they wouldn't show in front of customers. Or just think of it this way, don't you act differently around your friends compared to your family? I don't know about you but I cuss and whatnot around my friends but would never do that in front of family. So he's going through that same sort of thing. If it bothers you so much though confront him about it.

The reason for the suddent change, is male acceptance, and its "in group" vs the outcast, the reason he keeps training you like that... i cannot answerthe only thing I an say is stand your ground, but doing this can hav either positive or negative consequences

My fiance says that l can getpretty rude when we're in bigger groups of people, but they're not offended by it, as it show I've always been around them. It's probably a combination of that and the fact that it could be making him a bit nervous and isn't SURE how to act with both you and his family around. Guys are So much more worried about impressing people generally. It could be That he's embarrased about it and thus gets defensive and treats you the way he does. Try and give him the benefit of the doubt while still keeping your own happiness in mind. Hope this is of some help, sweetie. :)