Overly interested adults can be killing for a teenager's evolving hobby. I talk from experience. I too got into photography at 13 - the first hobby that I could explore by myself without family members constantly nagging me and pushing me in directions they personally thought was interesting.
However, I was thankful when adults would give me things I wished for for birthdays, like film, filters and money for developing etc. I seeked the information I needed from library books and archives of old photo magazines - your nephew will probably use the internet.
My advise is to support from a distance and also give color film and some mailers (if such a thing still exist). The idea of a skateboard photography book is great. you could consider adding something that exposes him to a wide range of photographers such as
http://www.amazon.com/PHOTO-BOX-Rob...UTF8&qid=1346742774&sr=8-3&keywords=photo+box
It is a while back I flipped through it and it may contain some nudity and violence (but probably not more than what you see on the new and internet anyway)