I suggest a different approach (or in addition to getting a workbook from a book store).
I suggest you get some interesting books (fiction and non-fiction) at a slightly higher level than he currently reads. So if he reads second grade level then get some at a low third grade level. Read to him a bit every night and every so often stop and ask him questions. Then ask him to read a page (or paragraph or 1/2 page) and he asks you questions.
Also my family has book club like activities. My child will read a book independently and be so excited he wants to talk about it. So my dh and I will read it too and then over dinner we will have something that looks like a book club discussion. Generally my son leads the discussion. It is fun to let him pick the food that night too. You might even be able to print off discussion questions off the internet.
A workbook is good for teaching a kid the types of questions they will run into on all the tests they will come across but nothing is better than reading together and helping him in the minute and shared interest in the story/material.
