To play devil's advocate, I wonder what he thinks of you? Giving him the benefit of the doubt, he could be in an even worse situation, loving to dance but feeling like his mother/teacher and partner think he is not good enough. I'm sure he knows you think he "sucks" and if he argues with his mom on the dance floor, they probably argue more at home as well.
I am speaking from experience that it is more difficult to teach or learn from a family member than it is a complete stranger.
Before you chuck him, why don't you informally get together with him outside of practice. Tell him you are excited, but stressed. Ask him what he is feeling. Maybe you will see another side of him. Hopefully, he will feel more "understood" and will not rebel by acting so immaturely at lessons.
He could have a low self esteem, which seems obnoxious, but is very cureable.
Can you have an outside coach work with you? A fresh set of eyes (outside of the family) my be beneficial.