Help! My cat is 15. He's not used a litter tray since he was a kitten, but he had surgery today and isn't allowed out for 10 days. How can I help him to use the litter tray? He's pining to be let out so obviously needs it. I've tried gently showing him the tray and putting him in it, but he's not keen. He's also obviously stressed and in pain which isn't helping....