Vocabulary Help

This holiday break is very welcome, I need time to really decide how to handle D's reading work. It's really difficult to figure out how to help someone increase reading comprehension. One thing that is helping is giving him vocabulary words to learn, very slowly, only two per week. He could handle many more, I'm sure, but this is working so well that I don't want to mess it up. (He's also learning Spanish so there's a lot of vocabulary there too.)
Something I've noticed is that he keeps getting stuck on idiomatic expressions, ones that I take for granted. DUH! How would he know them unless he's read them or heard them before? Somewhere in the past I had a book, or my parents did, that gave the origins of many of these expressions. I LOVED that book! Of course, it was many years ago and I have no idea what it was called, a quick look at the library didn't net me anything that looked like what I wanted. I need to look again more thoroughly because I think he would enjoy it. Online I found an article about the origins of language; he had asked where language came from anyway, why are they different, why are some the same. We're using the article for typing practice.
At CurrClick, I found a book called Workplace and Career - Vocabulary in Context by Saddleback Publishing; it's one of a series of books dealing with vocabulary for specific life situations. Two of them will be helpful, the others are too simple. Well, not necessarily simple, but he knows the words in them already. The only problem I see is that when the books teach the difference between formal and informal use, they use very out-of-date slang! The two books I want don't seem to have that.