Although I try to avoid Walmart, a gift card is a good idea, but impersonal. Some children struggle with reading so a book might be a huge frustration for a young teenager---instead he might enjoy some music.
I would suggest some shampoo with conditioner and shower gel (3 in 1 combination). My teenager loves this! Maybe a new backpack, gloves and a "cool" hat---one with those braid things hanging down. Love the candy bar idea. He would LOVE one of those fleece blankets that you tie all ends on... Notice that you are in a cold area. These are all things that my 16 year old has loved over the holidays.