Get a big jar or other container and fill it with these-
Little plastic toy animals/figures/packets of collectable stickers then surround them with sweets and other edible treats. Buy a few things each week and by Christmas you'll have a fabulous jar of excitement that he can put in his room and forage through at will. Discount stores are great for little cars/toys/ crayons/packets of art stuff and stickers....
Wrap it up with ribbon and paper.
Look online to www. BookPeople.co.uk/. They have great discounts such as a set of 3 rastamouse books for £4:99, 10 Moshi Monster books for £9:99, Lego pirate brickmaster book and 160 bricks halfprice for £9:99. Divide the book sets up into lots of parcels as that is what is exciting, not so much the contents.
The Big toy shed site has Hot Wheels Colour Shifters Piranha Attack Playset for £18:99 if that's any help
www.bigtoyshed.com/view-product.php?category=&product=2132
But keep a close eye on Argos and other retailers as they will all price competitively in the run up to Christmas.
also, our local newspaper runs a scheme whereby local people are asked to buy gifts and donate them. Local organisations then hand them out to families who have found themselves financially challenged. Can you see if your area has a local scheme like this? I know it can be hard to have to ask for this but please try, people want to help.
Does he like art? If so, you could build up a really interesting and bright art set by buying little bits each week- paper in different colours and types, crayons, felts, glitter, scissors, glue and collage bits. Find a container or box to arrange them in. Poundland has really decent art and craft supplies and don't discount independent art stores- they often discount ends of lines and have lots of little bits and pieces.
Look on Ebay for toys and look for local sellers who you can collect from. Gumtree is a good source too.
Don't forget cool little things for school that need not cost the earth- pencil cases, lunch boxes, gloves and socks, all these things can be sourced on sale at the moment- Paperchase aren't the cheapest but they do good sale discounts. And sets of things can be broken up and wrapped individually.