DSS always gets a couple of books, last year he got Teenage Kicks (101 Things to Do Before You are 16) and this year is The Teenager Who Came to Tea (brilliant spoof of The Tiger Who Came to Tea).
Accessories for his phone such as portable charger or light up charger cable.
Novelty USB sticks.
American/Japanese/generally foreign sweets, especially those with funny names such as Plop (you have to love teenage boy toilet humour).
Touch screen gloves or earphone beanie hat.
Sam Farmer does a really nice toiletry set that isn't strongly fragranced and has funky packaging.
Miniature aftershaves, can often be picked up cheaply on eBay.
Decent stationery such as post it flag markers for revision, mechanical pencils, erasable highlighters, Tinc electric eraser etc.
Plaque disclosing tablets (very silly but he always loves them).
Xbox points, iTunes gift card, cinema gift card.