Great job on kitty using the pie plate! Hopefully he now has the idea and can graduate to a big boy box. Is it possible to leave a box in "his" room for a while? That way he will remember where to go and will return there. It's possible if you move the box too quickly he will go to where it was and use that spot. He is just a baby and will not always remember.
It is not always advisable to have boxes side by side. If the kitties do not get along and they have to use the boxes in close proximity of the other it is possible they will revert to using other spots.
Keep up the good work!
And there is no such thing as posting too much!!