How does your fiancee feel about the plan to send him to his father's everytime he acts out like this? Is his father willing to take him at a minutes notice despite the presumably court ordered visitation schedule?

I do agree having family counseling could help these two work through this. No sense in giving up on a child that is letting their mom know they aren't in a good place.

Why do you think it is that he's only like this with her? What does she currently do that isn't working?