I would explain in real simple terms ... suggest he get into a Target Date Fund. Then you could discuss the differences in the various Target Date Funds' make-up (e.g. mostly large U.S companies, some international companies and some bonds). The two of you could pick a specific fund. You could talk to him about ways of adding to the fund. Then, if he's interested he could start reading. Hopefully, he won't be all that interested--and that he just continues to put money into the Target Fund (ok, maybe something into Wellesley as he get older).
I second Redduck's idea of the TDF to just get things going, assuming it's index-based and low cost. But I'd also recommend a book to help him determine a suitable AA and choose the TDF accordingly, rather than based on date name.
I've read - and keep in my library - nearly all of the books suggested here. I like them for different reasons but - when recommending a single book for someone starting with zero previous knowledge in investing - I go for drop-dead simple and a quick read. I like Dan Solin's books for that reason and others. Quick read, short chapters that get right to the point, covers the essentials, how to do it with Vanguard, Fido, or TRP. By the time the reader/new investor has time to get bored or sleepy, he's already at the end of the book and with the most critical basics grasped.
Mike Piper's book - mentioned by others - is another great choice. As much as I like 4 Pillars, Random Walk, and others, I feel they are more suitable for a few weeks or months into the new investor's learning process - just not day one.