Are there any food things he likes that you don't normally get him that come in big packs that would be a treat but would also take up lots of space?
things like the star wars cereal - apparently it comes with a special spoon or toy at the moment or something - going to investigate tomorrow. But it's something I wouldn't normally buy the dc for their breakfast - so they'll be chuffed if a box appears at christmas I reckon, plus it's (hopefully!) a big box rather than a single portion size, so will take up space.
new comic or magazine he'd like and one about the raspberry pi
new football or rugby ball if he'll use it (or tennis or cricket or whatever his thing is)
a rubiks cube that is bigger than 3x3 - so say a 5x5 one.
some cookery stuff or art stuff (the works or Cass are good for cheap but not childish art stuff)
shower gel or pants, socks, toothbrush, etc etc - boring stuff but maybe a bit more fun than usual everyday stuff
something a bit silly to have fun with (a blown up balloon with BOO or some such written on it and wrapped up)
personalised jar of nutella
ds1 is 10 and adores having a cosy blanket - asda have really nice microplush ones for about a fiver that last really well and are really soft to snuggle up into or Tiger have fleeces throws that are marked out like a football pitch for £4 - both would take up plenty of space!
is he too old for a personalised match attax card? some boys seem to adore them even as they get older, others never get into it or have them as a primary school thing...
emoji cushions can be cheap on amazon - this sort of thing although I hadn't realised quite how many poop emoji cushions there were - not sure I fancy those! random emoji cushion example - I haven't searched for best price or delivery sorry
go and have a look on the current christmas bargains thread in the christmas section - think it's up to thread 14 for this year so far... lots of ideas and bargains!