I would suggest going to Khan Academy http://www.khanacademy.org/ and scrolling down to Developmental Math. There are quite a few videos which this child could watch over and over until he gets the concepts he is having trouble grasping. Then I would go to Dad's Worksheets http://www.dadsworksheets.com/ and print out the appropriate worksheets for him to use. Another online site you might want to check out is TenMarks http://www.tenmarks.com/ and set up a free account and add him to a class you create. Then he could work on his math on a computer and you would get detailed reports each week about how he is doing.
Hope this helps and good luck.