While such questions are personal to the values of the parent, and the abilities and maturity level of the child, there is a "rating system" for books as well that at least helps to determine what a child is likely to be capable of understanding. Many books are registered with an "RL" on them that can be found near the bar-code or on the copyright page. If not there, you can often look it up online through the publisher.
In this case, the RL is 8.3 (Scholastic). You should be aware that an RL is an indication of what average grade level (in the US) will understand at least 80% of what is being read - in this case, an eighth grader about three tenths the way through his/her eighth grade year. It is not a maturity rating in terms of content. It also may not be helpful as grade level is different in other countries, but there is a more internationally minded answer to the question as well.
Lexile measures are another way to gauge the difficulty level of reading in a book and measure approximately the same thing - they just go about it a different way. This book is measured at 1000. For the sake of comparison, The Hobbit is also measured at 1000L by the Lexile framework.
In terms of determining specifically whether your son is ready or not - it will depend much on how his reading level compares with that of the book. If you look up the Lexiles for other books he has read and understood successfully, you can determine if his score would be above or below 1000L (my guess, is that it might still be a bit early, but if he is an advanced reader and is fairly politically minded, maybe not).
This will not help you in gauging his ability to grasp the background needed for some of the social commentary and jokes. But if his reading level is generally above an 8.3 he stands a much better chance of understanding than he would if he would score below it. This is where the gray area comes in and you, knowing your child best, are the best one to determine if he has the necessary background in politics, culture and Science Fiction to grasp the humor. As a suggestion, whenever I am unsure if my child is ready or not for a book (but I think she is close), we read it together. It is a fun way to bond, we can discuss what we are reading if she gets stuck or doesn't understand something, generally, they understand what they hear better than what they read, so you can read the passages that contain a lot of political jokes (or other things he might struggle with in terms of understanding, and it allows me to remember all the details of the book and enjoy it a second (or third, or fourth . . .) time around.