I'm starting to wonder whether people may wonder whether I'm on commission, but I wonder whether you've looked at ALEKS (
www.aleks.com)? Your teacher sounds as though she's on your side, but you say the extra work she's providing also isn't at your DS's level, which makes me think whether it might be useful if you could send in ALEKS worksheets instead. (There is also EPGY with the open enrollment that melmichigan runs, which many people like better, but which doesn't AIUI let you produce worksheets customised to the individual child - sounds as though in your case that might be the really valuable thing.) If you haven't looked at it before, you might like to try their free trial and see what your DS makes of it.