I would just get a few stocking fillers and then give him cash.
16 is a difficult age when young people, boys in particular I think are often changing their 'identity' , growing up, taking on new experiences as they start college, get girlfriends etc etc
Age 15-16 my DS was very much into sports, fitness, healthy eating, only wore tracksuits etc
Age 16-17 he went back into computer games, spent most of his time listening to music, going to gigs, only wore band T shirts
Age 17-18 started work, going out drinking, has GF, likes to wear brands
So thinking that although cash seems a bit impersonal it's likely that over coming months your DS will probably develop interest in some thing which he needs money for and better to have it there than waste money now on something he doesn't really want. Is he 16 doing GCSE or post GCSE?
I also like the gift cards eg for Nando's, cinema but depends if your DS would use them. Clothing eg pyjamas, sweatshirt with logo of something that interests him? I think if he does have a sport that he is into I would get some thing small around this, maybe some thing small merchandise around the game he plays, chocolates and then cash as there will definitely be a point in the next year or so he'll be glad of some savings.