Put another litter tray upstairs, at first in the wardrobe where he poops, and then gradually move it to where you would prefer it to be. You could also try changing the type of litter you use, And get a vet check in case defacating is causing him physical pain, and so he is associating the litter tray with pain. Also I find the supermarket and pet shop litter trays too small and shallow. I use big 25-30 litre plastic storage boxes, although you might choose something smaller for a kitten. They like to dig, and a deeper container lets them do that.