Did you try unplying the yarn and sewing with that? (Sometimes works, sometimes not. Depends on breakage factor.) I usually opt for trying this before opting for sewing thread. If you have beeswax....those of us who tailor rely on it...rub the thread and that might make it sturdier AND provide a bit of coating so the thread can't be as abrasive. Regardless of the sz of the sweater, I always use backer buttons to take tension off the working button. (Esp, in this case, where you have one button that'll be taking all the stress.) I'm sure you make shanks where needed. Might be the case here, as well. That helps relieve some of the 'working' of the button.