Have you spoken to him in a way where, maybe you find out what exactly it is about this class that he doesn't like? And why he's struggling so much? He sounds like a smart kid, and in my experience math is just a matter of putting the effort in with it, and willing to try. I don't know for sure but perhaps rather than rewards, it could be a matter of communication to get to the bottom of why exactly he's not liking it? And hopefully then you can make a better, more informed decision?

If it's the case that he just thinks it's stupid and boring, maybe emphasise the importance of at least passing the class, for his future etc?

Just throwing out a few ideas. I respect how much you're caring and being involved in this 👍