A.L.
My son and several of his classmates (6th graders) have read and loved the Divergent series. Based on my quick glance at them, I would say that the content is definitely young adult. It is a 'dystopia' book (imagining a really nasty society in which to live), so if your son is squeamish or easily disturbed, I would not recommend it at this point. Then again, if he wants to read a book about boys getting lost in the wilderness and barely surviving, he is probably not easily disturbed. As far as the sexual content goes, there is some lusting by the main character and a bit of kissing, however my son currently labels romance stuff as 'disturbing' and he loved the series, so clearly there isn't much of it. I think you should give him a chance to try it out. If it isn't appropriate for him now, he'll self-select and stop reading it himself.
I do second the recommendations for the Percy Jackson series or the other series by Rick Riordan (The Kane Chronicles) as well as the Eragon series. Our son loved them and even though he has moved away from fantasy, he still eagerly awaits the next Rick Riordan book's releases. Good luck and have fun!