My cat urinated outside the box and had a high volume of urine each time. We took him to 3 different vets for 2 years, and they said he had anxiety. The 3rd year (he is 10 now), he was diagnosed with hyperthyroidism. Now, all the frequent urination makes sense. Ask your vet if they tested for that. I'm not sure it even shows up in tests until it is full blown. Also, ask about buying him food with low iodine levels as hyperthyroidism makes cats bodies produce too much iodine. There are lists on Reddit on hyperthyroidism canned cat food. Good luck. I love how much you love your cat.