Is he too young yet? I think for as long as I remember I have asked him to tidy up after himself (with help when necessary, of course), and he's always been very keen, in fact I think he began tiding before I started asking IYSWIM.
However, recently it's become a bit of a battle, and I've started wondering if it's too much to ask of him?
He's about to turn 4yo, a week off.