You might like to pass on Robert Muchamore's "Cherub" series to him. I've only listened to a few on audiobook because they were read by one of my favourite actors, but the stories seemed to hold up pretty well.
They're about teens being recruited for spy missions, but they seem to be geared a bit more towards older mid-teens with slightly more serious plotlines (and maybe a bit more violence).
There's a nice
official website here with a few free short stories that you can try out and determine if they're at an appropriate reading level for your son.