Parent of teens here.
I have no idea of your community, options for shopping, local entertainment and activities, transportation etc.
My son would love any of the following:
Mall gift card, this is better than a card to a specific store.
Gift certificates for 2 to go to the movies, go carting, paint balling, mini gold/driving range, you get the idea. Two gift certs, so he can take a friend and have more fun.
Book store card only if it was to a big box store that sells much more than just books. My son is a nerd and would love books, but his reading is far advances than most kids. His best friend would be upset to get a bookstore gift card, as he has real difficulty reading and he would feel bad to get a book for Christmas.
We used air miles to buy expensive electronics (MP3 players), it was not money out of our pocket and very much appreciated.
If you have transit locally a bus pass would be a nice extra.
Please, please do not buy the kid socks, or necessities. The idea is to give him a gift, not something he needs to go to school or clothing. Unless you are making up a stocking, but make sure there is a gift that is not a necessity.
Thank you for thinking of a child in need and making his Christmas special.
I worked for a NFP last winter and we gave 3 families Christmas gift cards. All got bus passes as well as the gift cards, so they could get to and from the stores. One family was given a taxi pass, as there were mobility issues and no way they could get shopping home on the bus.