I suppose it might depend on his interests. I always liked something by Tolkien or Dostoevsky, but I think you’re asking for books that would be labeled explicitly Catholic. I don’t really know if I should be recommending this, but I found it vastly amusing to read all the same. Sicola, the Papal Bull: A Fantasy on the Escapades of an Itinerant Pope. It’s a novel by John A. Vitello, and is fiction, fiction, fiction. Sicola’s adventures are divided by chapter, and are intended to address changes in the way Catholics act since, well, arguably more recent changes. It’s rather hard to find in a bookstore, and you’ll probably want to read it yourself before handing it to your son, but it’s very easy to read and there is space for a good chuckle or two as long as you don’t take anything too seriously.