Thank you for all of the suggestions and ideas. I ended up buying an adjustable height desk, along with a couple of child sized chairs. For things such as handwriting I felt that it was better for him to have a table with the right proportions, but I'm not really fussed where he works for other subjects.
He loves working on the floor for maths, whereas I preferred to work sitting up in bed when I was a child! It just goes to show how different we are :)
OhElizabeth, we both have a genetic disorder which affects our joints, causing weakness, pain and dislocations (amongst other things). The issues with handwriting are definitely genetic in nature for us, so I should probably look into an OT or physio for his hands. Unfortunately there isn't much that they are able to do to help, but having been there myself, I know how much pain handwriting has the potential to cause! I suppose it is just one of those things that we have to learn how to live with and to adapt to.