[Ronin]

Yeah, I don't think Ultimate Spider-Man ever gets too adult.

There is violence, and the occasional character death. I can think of a couple moments in the whole run where adult relationships are alluded to, but nothing really explicit - it's implied that Wolverine made some moves on Mary Jane at one point (there's a significant age difference, but Ultimate Wolverine is a creep) - but the specifics are kept pretty oblique.

I think if the Marvel movies are okay, Ultimate Spidey should be okay for him as well. It has been a while since I read it, though.

Just be advised that like Spidey says, the other Ultimate books tend to be a lot less kid-friendly. Ultimates in particular was written by Mark Millar at his most unnecessarily 'edgy'. So if he gets curious about characters from another book showing up, that other book might be a no-go zone.