Cats are delicate. I'm not sure I understand but regular tuna is high in mercury. Isn't good for cats. There are lots of good canned food on the market now. If it's not the food, better take kitty to vet.
I've never heard a cat cry out when making their BM. Maxy
If kitty is crying while using the litter both, something is going on in the gut that a vet should investigate. Tuna is fine as a very occasional treat, but it is not complete food for kitty.
Think it is very important you get your kitty to a vet. He is in pain , thus the howling in the litter box. Poor thing!
Please take him as soon as possible and let us know what the vet says.
The Tuna and his diet can be straightened out when you talk to your vet. Cats need food especially made for cats. They need things like Taurine which they will not get in people food.
Please keep us posted.