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:
Point is, whatever it takes to get students actively involved with the reviewing process where they are not bored and effectively reviewing grade level material in order to prepare them for state or quarterly assessments.
There are other sources for worksheets also. You can find many public schools and private schools which will provide free worksheets for you if you buy textbooks from the school.
Subtract 1-, 2-, or 3-digit numbers from 3- and 4-digit number with/without renaming