It's not Mark Tufo, he wrote "the other Zombie series" (which is good and has an element of fantasy about it too - apart from zombies being unreal that is! ) - White Flag is by Joseph Talluto. I always get those two mixed up
Heinlein. Although they are dated (sort of) now, eg - they have mobile phones which to a modern reader means nothing, but when they were written was Far Future sci-fi iyswim, they are exciting, and all his science is correct. Say he explains how they spin a spaceship to get artificial gravity, well his math is there and will be correct, if you built it, it would work. All his protagonists are interested in science and maths, and work and study hard - it sounds naff maybe, but they are all "good role models". Depending how you feel about him reading about relationships, get the juvenile novels. The adult ones, although not exactly explicit, do have a fair few "unorthodox" (at the time they were written) relationships and a good bit of sex in them - they aren't porn, but they are more adult than the juvenile and short stories. That said, the constant theme is of loving relationships and raising children together and "properly". Starship Troopers is a good one, or Red Planet, Citizen of the Galaxy, Between Planets, Starman Jones - they are all in the juvenile novels. If he liked On The Road and wants more dystopian/post apocalypse type stuff, then John Wyndham and John Christopher have written lots - the only one I would say avoid is "the Pendulum Swings" by JC, otherwise everything they wrote is good - not too violent or bloody, but well written and with great story lines - again JW is a tad dated but loses nothing for being so. Louise Lawrence and Robert Swindells also write good post apocalypse stuff, although some of RS's are aimed at younger readers. Guy Gavriel Kaye writes fantasy stuff, if your DS enjoyed LOTR. Or Raymond E. Feist's Magician, Silverthorn, and Darkness at Sethanon trilogy, and the "Empire trilogy" are both brilliant. They're the first six of the Riftwar Saga, and there are lots of them in the series, but those first six are MUCH the best. If he wants zombies, the "White Flag of the Dead" series by Mark Tufo is brilliant, much better than the run of the mill "dawn of the dead B movie" type stuff.