Might sound wierd but, have you tried putting the feaces deposited outside the litter tray into the litter tray so that next time the fur-child visits, he will see where it is meant to go? Clean it afterwards as you normally do but make a point if letting him see/smell his own 'doings' inside the litter tray - may have to do this more than once but your cat should get the message pretty soon. Its how I train my siamese kittens and they pick it up straight away but with a cat who has always poo'ed outside the tray, it may take a few goes (pardon the pun).

All the best.

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kind regards

Ra
http://www.bobergsiamese.com/