I would also recommend both Hitchhikers and Terry Pratchett.
As Apile said, I think "Mort" is a good place to start; however another good place to start is "Guards Guards!" which sees the first appearance of Carrot Ironfoundersson.
Just looking at something slightly different though, there is the "Dangerous Book for Boys" by Conn & Hal Iggulden.
This is a non-fiction book and it's about going outside and doing stuff rather than sitting in front of a screen. (I'm really old so I remember the TV show from the 70s and 80s called "Why Don't You...?" - it's sort of got the same feeling as that).
Have a read of the reviews on Goodreads here:-
www.goodreads.com/book/show/204097.The_Dangerous_Book_for_Boys
Here is a typical review:-
"I bought this for my little brother's eleventh birthday. I knew my parents were finally breaking down and buying him video games, so I thought this would be an appropriate gift. Poor kid, it was already well-thumbed by the time I finally got around to giving it to him. Yes, The Dangerous Book for Boys can certainly be enjoyed by twenty-something girls, too."
"It's crazy informative. Do you know how to skin a rabbit? I do now. Or how to build a go cart? Race ya. Or what went down at the Battle of Thermopylae? Or what an objective pronoun is? See? All very useful information. It's also pretty darn funny. It advises young boys that when they're older, flowers really do work with girls. But it warns not to try it now--they'll just look awkward. You get the idea."
"Basically, this is the best book ever, whether you're a boy with a long summer ahead of him, or a grown-up who remembers what those summers were like."
I would say, however, that is seems to be aimed at sort of a 11-13 age range so I don't know if your DS would be too old, but in event, it's definitely a good read.
Just talking about Conn Iggulden though, he's also written a whole load of fiction that your DS may well enjoy a lot.
He's done a series of books centred around the life of Genghis Khan starting from when he was a young boy (Wolves of the Plains) through to his death and his children and grandchildren Ogedai and Kublai Khan.
He also did a similar series charting the life of Julius Caesar nfrom being a young boy (The Gates of Rome) until his death.
Both series of books are full of lots of action and adventure and both series of books start off from where the protagonists are young teenagers and follow their lives from there.
I have known quite a few teenage (and older) boys captivated by these books - and the other later books that Iggulden wrote about other famous historical characters as well.