He's in luck! There's a lot of that in our house! When we got back, I told this to Mom, who said: "I have every one of the Bolo series including an original edition of the first Keith Laumer book! I think he'll like those."
Me: "There's the 'There Will Be War' anthology too."
Toly: "And Hammer's Slammers and anything by David Drake!"
Dad: "What about the Honor Harrington series?"
The upshot is that AD now has a lot of reading material of varying levels of adultness.