Some travel sized toiletries of his favourites, or some more "grown up" type ones, possibly including hair gel or mud to experiment with, or pack of a mix of small sizes of shower gels in different smells to try out....
A magazine on an interest he has - not crafts, but does he like games, or a particular sport, or current affairs, or history...…
A slinky
Shower radio
Tools for fixing bike etc? Wallet card tool, or a swiss army penknife (if you teach him how to use it safely)....
Favourite sweets or savoury snack
Pocket calendar
Accessories for phone/gaming device etc, pair of earphones, spare charging cables or power bank (if anything like my teen girl, these always go missing/broken so spares are very useful!)
"Cool" mug (in his kind of style - whether a regular pottery one for at home or a thermal one for out and about) and a favourite hot drink for it, or a reusable water bottle
Carbeeners (even if not into outdoor pursuits, carbeeners are great for hanging things to bags etc as teens)
Small puzzle book and a refillable pencil (the ones where you can refill the lead) or small sized pen
Bits for any hobbies - guitar strap and plecs, music sheets, gaming token, studs kit for boots, mouse pad, some fun long socks not just school/club colours…
Some more fun than everyday utilitarian socks and underwear
Cool hat and gloves
Bamboo toothbrush, wooden hairbrush, other eco friendly toiletries
Wallet
Travel accessories - pouch of cutlery for 1, reusable toiletries bottles, travel towels,
Voucher for the cinema
Kit to grow crystals or other small science kits
Story cubes
Cards or small games (travel, one person, or just small sized so good to bring to friends houses)