This will probably sound odd..., have you thought of taking some of his Poo with you on a walk ( wee a bit difficult ) dropping it somewhere suitable for him to smell, it might just do the trick.
When Bea was a pup I didn't clear up after her straight away in the garden, she would then smell where she last went and do it again close to that area.
It shouldn't take him to long to cotton on, especially with the key word and loads of praise.