Hi again, thanks for your reply with additional and helpful information.
There are various possibilities, but the top ones are: he ingested a toxic substance outside (or in the house) and this is causing his symptoms. If he had any access to rat poison, or is a hunter and kills/eats mice or parts of mice or other rodents, he might have been poisoned 'by proxy' with rat poison, which causes uncontrollable internal bleeding; it removes the clotting factor from the blood and is very dangerous, requiring immediate vet treatment.
If he has not urinated today, he may have a urethral obstruction, which means he cannot pass urine out of his bladder and the toxins building up in his body can cause severe illness.
He could have been injured outside by a car or other vehicle and is bleeding internally.
As you can see, there are so many possibilities, but considering the symptoms you described, it would be advisable to have him evaluated in person by an ER vet, immediately.
I hope all will be well with your furry boy once he is assessed and treated.
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