You primary objection concerning this book relates to the violence portrayed, and the themes of death. However, what you fail to take into account is that fact that 'death and violence' as a theme is one of the most comment place themes in the world today. There is no denying it. Even the most innocent of children's tale - the traditional tale of Cinderella (involves child abuse), Jack and the beanstalk, ( both involving murder and cannabolism), Snow-white, (murder arrives on numerous occasions), and the more modern feat of literature works, such as the Chronicles of Narnia, which involves numerous battles, fights and deaths.
In the works of literature, and film, along with television pro grammes there is no escaping the fact that these are just fundamental elements to stories. If however you are of the opinion that none of the above works are suitable - take a look at any religious text - it includes violence and murder. There is no escaping it. What can however be toned is the context and the strength of said images. This has been a major point in JKR's work of Harry Potter, and thus their prevailing status as children's novels.
I have to state however that I don't think you should be concerned about a 13yr old and his delicate mind. If you took a look at what 13yrs olds talk about in the course of their school day then you are in for a shock - they are not only adult in their language and concepts, but use them most ineptly. I also can not help but notice that you comment of 'seems violent to me' seems to be off hand and with out conviction, ie you would like to start a heated discussion on this forum.