That's great that you're helping your nephew start a comic collection. I did the same thing for my nephew, who is 7. He started off with Batman: Brave and the Bold and then expanded to Marvel Universe Ultimate Spider-Man. I think any of the Nu52 Batman and Superman titles would be a good enough fit. Some of the early issues of Detective are a bit gory, but nothing worse than what's on tv. In the Batman titles currently running, they are talking about the early days of Batman and how he got started. So it would be pretty easy for him to pick up Batman there. Just look for anything labeled "Zero Year". Superman just started a new arc as well. Plus there is the digital first but available in print Legends of the Dark Knight and Adventures of Superman. They are both essentially a collection of short stories about Batman and Superman. They are stand alone works and you don't have to know anything about either character going in. He may also like older issues of Superman/Batman or the new title Batman/Superman (which is only a few issues in and a new series). You can get the first couple of story arcs in collected trades for the Nu52 Batman and Superman titles, so that may make it easier to get the stories. Really though, taking him to a comic shop or book store is the best way to go. That's what I did. My nephew picked out what he liked and we got to have a good family bonding experience. Plus the guys who work at my comic shop are very nice and helped suggest what my nephew may like. So it was good experience all around.