You're a teacher challenged with a difficult student. You've tried being nice and not so nice, going back and forth between these extremes. Tough love doesn't vacillate. It's not an oxymoron.
Tough love is a practical, steady attitude.
A math teacher is never even tempted to accept a wrong answer. Every right answer is a foundation on which to build and every wrong answer is an opportunity to problem solve. Let no problem allow you to forget you're a teacher. It's the difference between supplying fish and teaching one how to fish. Instead of attempting to force subject matter upon a student, create a sense of value for and love of learning.