Casting my mind back 4 years LOL ....
A book of bad/corny jokes (for some reason 13 year old boys just love them, but be prepared to be regaled with them all day
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)
Hair gel/wax
Science Museum Silly Putty
Hand Held Puzzles like these https://www.handheldpuzzles.co.uk/?gclid=CLP7lNrv7aQCFVH-2Aod81Q20g
Some Tricks/Jokes https://www.jokeshop.co.uk/cgi-bin/sitewise.pl?act=sect&s=2855&id=jokeshop (again - very 13 year old boy
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but be prepared to suffer them all day)
Gloves
Anything to do with fave football team (if into footie) ... mug/keyring/scarf/boot bag etc
Anything to do with Simpsons/Family Guy etc (dvd, keyring, novelties, socks, puzzles, mug etc)
A 'Do Not Disturb' door sign
Puncture repair kit/water bottle/spoke discs/multi tool etc if he is into riding his bike
Wind-up Torch
Mobile Phone/Ipod Accessories if he has either of those (covers, headphones, socks, iTunes voucher etc)
Poster for his room
Sweets