C.C.
My thought on this is that you should not attempt to find workbooks that do math the way they're teaching it in schools now. The way they're doing it makes no sense to anybody. He might be better off just learning how to correctly solve the problems (and then when the school year starts again, he can pretend to solve the problems whatever cockamamie way the teacher wants him to ;).
My kids really love the Life of Fred books. They're technically meant for homeschooling, but they are *fantastic* books for any child to do outside of school. My daughter will also be going into 5th grade, and she is starting on the Fractions book - but if your son needs any review, you could start with the intermediate series. He will enjoy them no matter where you start in the series. Here's a link to some sample pages on the author's website: http://www.stanleyschmidt.com/FredGauss/31samplepages.html