I'm not sure what sort of books a 13 year old boy would like sadly (my 9 year old daughter only reads graphic novels) but there are a couple of book sharing comms on LJ if you are still over there at all and can get in them... bekeen and v50 They've slowed down a lot since demonoid went down though.

Also yes you need mobi files but you can convert other formats to mobi if you have calibre (http://calibre-ebook.com/) (which is free and easy to use) It's also good for organizing one's digital books.

good luck. The holidays are so stressful when there is no money :(