Remember to select worksheets that are the right level difficulty for your child. Get something too hard, and your child will become discouraged. Make it too easy, and they won't learn much.
Once you have a scope and sequence book, make a list of each area in math that he needs to work on for the school year. For example for grades three and four, by the end of the year in subtraction, your child should be able to:
How much money do I have left if I buy a soda? By the end of the week, how much of my daily allowance will I be able to save if I don't?
Review subtraction of 2 numbers whose sums would be 18 or less