Yes that is hard! i would probably mainly go with foodstuffs but you'll know what he can manage. Other top of head things:
Seeds for garden Mini grooming products like shaving foam, spot cream etc Shoelaces Funky hat Skateboard or bike stickers Chapstick Socks Pants Squash ball Novelty Top Trumps Star Wars Lego keyring or light iTunes voucher (agree with other poster) 2014 tear off desk calendar Mug Hot chocolate swizzle stick Mini cookbook Artists pencils Tuning whistle Satsuma Hook for bedroom Funky doorknobs for bedroom furniture Wall stickers
Kind of depends on his bent/needs and how much money you are wanting to spend (I reckon I racked up 'shit loads' for you there ).
Last thought, if you google miracle berry tablets, they are a tablets that changes how foods taste (totally harmless!) They have them on amazon, I think. You'd have to cut a strip up for them to fit, though.
Agree it's much harder to find things small enough for boys. You could wrap some bigger things and put in a gift bag then put a 'lucky dip' card in the calendar? You could do for DD too to fill any gaps. Is DS an older or younger teen?