There are various schools of thought on this - some say it's just they like the taste, some say it's a habit, others say it's a deficiency in their diet. Mine will pick it up thinking I'm not looking then run off when they see I've noticed, so i never catch them actually eating it. They know I don't like them doing ti but can't stop themselves. If it's outside in the fields and rabbits/sheep etc I just don't look, but I don't like them eating cat poo as cats pick up worms from hunting all the time. Can you keep the litter tray somewhere he can't reach it? or get a covered one?