Everyone else has covered this... but my vote is that you can given him a book or two. He doesn't have to read it right away, but I'd be sure he'd peek later eh.

As long as it's not a kiddy stuff and doesn't talk down to him..

I'd be looking for something that covers
- relationships (and types of)
- sex differences and various opinions/theories off
- anatomy
- some 'soft' pictures/illustrations
- diseases
- different types of sexuality

Thing is... my concern would be that the material he finds himself, in todays world, might be just a little bit too hard-core. Best to get in earlier I think, with something relatively rational and wholesome.