In this bildungsroman, or coming-of-of-age novel, the main themes are homelessness (and its causes) as well as globalism. It is the main character, Ian, through which we learn about these themes.
The theme of homelessness (and the causes of that homelessness) is presented early on in the novel. Ian is only fifteen and is required to finish some community service hours at "The Club." He is aghast to learn that it is a soup kitchen serving only homeless people. Being in an unsafe neighborhood at an ungodly hour, Ian narrowly escapes a mugging because another homeless man saves him (by attacking Ian's muggers with a pipe). Ian is absolutely terrified and disgusted at his job at "The Club." However, Ian soon meets that same homeless man who saved him, finds out the man's name is Jacques and is known by all as "Sarge," and that he fought for years in Rwanda. Due to his service in the Armed Forces, Jacques suffers from severe PTSD (Post Traumatic Stress Disorder). It is this PTSD that is the eventual cause of Jacques' homelessness.
Globalism is more of a muted theme in the book, but is one that I think is very important in regards to the novel being, in particular, a bildungsroman. At the beginning of the novel, Ian thinks locally. By the end of the novel, Ian thinks globally. At first, Ian is only concerned with his own safety, how he appears to others and to his friends, and is intent on feeling disgust for all he meets at "The Club." It is Jacques who teaches Ian to think globally by sharing Jacques' story in the Armed Forces in Rwanda as well as the specifics about the horrors of that war and the effects of his PTSD. Thus, Ian learns many new things and struggles with how to help Jacques. Ironically, one of the big answers is that Ian can be a good listener for the PTSD afflicted Jacques. By helping him release these horrors of memory, Ian is able to help Jacques and many other homeless.
Ian and Jacques both prove that only one person can make a difference in the world and affect it greatly. Therefore, it can be assumed that the themes of homelessness and globalism come full circle in this novel.