Why the litter tray is irresistible to him
It helps to accept the unpleasant truth first: cat faeces often contain undigested protein and fat, and to a dog's nose the litter tray smells like food, not waste. Cats pass richer material than dogs do, the tray is at perfect nose height, and it is usually in a quiet corner where a dog can snack undisturbed. This behaviour, called coprophagia, is common, usually harmless in itself and extremely resistant to disgust-based training, because your horror does not change how it tastes to him. Scolding after the fact mainly teaches him to raid the tray when you are not looking, and chasing him mid-raid turns a mouthful into a prize worth grabbing faster. The plan that works is unglamorous: make access physically difficult, make the tray empty most of the time, and teach a leave-it for the margins.
Engineer the tray so the cat can reach it and he cannot
Choose a barrier based on the size difference between your animals. A baby gate raised just enough for the cat to slip underneath, a door strap that holds a door open a cat-width, or a cat door into the room holding the tray all exploit the fact that the cat is smaller or more agile. Covered trays with cat-sized entry flaps slow a determined dog but rarely defeat one, so treat them as a delay, not a solution. Place the tray away from his usual routes if you can, and avoid moving it so often that the cat starts toileting elsewhere in protest; cats are conservative about their arrangements. Test the setup by watching whether the cat uses it confidently, because a barrier that stresses the cat trades one problem for another.
Scoop promptly and train the leave-it
Every hour faeces sit in the tray is an hour of opportunity, so scooping as soon as you notice it has been used shrinks the problem dramatically. Teach leave-it away from the tray first, with low-value items and generous rewards for turning back to you, then practise near the tray on a lead so success is likely. When he does snatch something, trade calmly for a high-value reward rather than prising his mouth open or shouting, which can make him swallow faster next time. Keep his own feeding routine regular and his diet complete, and clear his own waste from the garden promptly too, since a dog practised at scavenging outdoors will happily extend the habit indoors.
Health points worth knowing
Raiding a household cat's tray is usually a hygiene issue rather than a medical emergency, but it is sensible to keep both animals' worming and parasite prevention current, since litter trays can pass parasites between pets. Mention persistent coprophagia to your veterinarian if it is new in an adult dog, comes with weight loss, a ravenous appetite, pale or greasy stools, or other digestive changes, because those combinations can point to problems with digestion or nutrient absorption that deserve investigation. Also watch for litter itself being swallowed in quantity, and for any vomiting, straining or appetite loss afterwards, which should prompt a veterinary call. Keep clumping litter and cleaning products stored as your veterinarian and common sense suggest, safely away from a determined scavenger.
Share the unglamorous job that fixes it
This problem is mostly won or lost on scooping speed and gate discipline, which are exactly the small recurring tasks that slip when they belong to everyone and no one. Put them on a rota: who scoops morning and evening, who checks the gate is reset after cleaning, who practises leave-it this week. In KinsLe, Care Teams lets you assign those litter and gate duties to named people with completed tasks marked by time, so the tray is emptied on schedule instead of whenever someone happens to notice it.