I imagine everyone has this in their collection already, but I can't stress how important it is often enough. Yes, decades have passed since it was published, and Douglas Adams himself is long dead, but the series lives on, as recently as 2009, when Eoin Colfer (ooh, a kids' book tie-in) wrote a surprisingly good 6th installment. If a boy who is transitioning from kids to YA/adult material tells you he loves funny books, and likes British authors, this should be the first stop.
I don't need to describe the plot here. I suspect everyone has read it already, and if not, you need to drop everything and give it a try.
The point I'm trying to make is that while I disdain "classics" in general as being too stodgy, too irrelevant to modern life, some are timeless. Teen boys will always read this. After all, the Guide is Wikipedia, and how modern is that?