DS1 has a desk but no chair

[FabIsGoingToBeFabIn2010]

He is nearly 9. What is better for his back, a stool or a chair with a back? Any recommendations would be great too though the budget is tiny unless I can sell a child some stuff.

Chair with back, Ikea do some basic height adjustable ones like this

I think swivel would be a mistake for him but thank you. I will look in Argos too as Ikea is miles from us.