@makingmiracles I can second the IQ smart games - my ds loves them and when he has them out (he often takes them to family parties so he can use them for a bit of escapism!) there are always lots of people that want to have a go, of all ages and abilities!
Along the same lines is a great game called Rush Hour, which is a traffic puzzle (there's a deck of cards wtih about 100 different puzzles to work through and you can get expansion packs too) It's £11 on Amazon at the moment which is quite a good price for it - last year I got it for £9 or £10 at The Range, which was the lowest I've seen it.
Sainsburys seem to be putting out their sale stationery in drips and drabs this year - I've usually been lucky in October time and managed to get loads of things with 90% off, so have stocked up for myself and as stocking fillers and presents for the dc at Christmas and birthdays - this year nothing in October particularly, but every week there seem to be a couple of different things discounted on the stationery aisle or end of aisle bargain area.
With regard to a present for a dementia sufferer - might be for someone who is in the later stages of the disease but lots of people swear by fiddle blankets made specifically for this. There are some on ebay and etsy, but might be worth speaking to somebody who knows more than I do as to where a good place to get them would be as I don't know the specifics around the quality, the 'fiddles' and price.
Alternatively, how about a photo blanket with pictures printed of loved ones and favourite places so he would always have them with him?
This is one from Tesco which is a bit expensive but it's the sort of thing that if you google it and look on Groupon and Wowcher there always seems to be somebody with an offer on for them from £10 up, so a lot more tractable!