This could be quite serious if he is trying to block. We see this very often in male cats because their urethra is much smaller than in females and it tends to become fully blocked by urinary stones or a urethral plug.
If he is still urinating some, I would recommend getting him to drink as much water as he wants, by offering fresh cold water often. A vet visit will be needed tomorrow so his urine can be checked for blood, bacteria, and/or crystals. He may need x-rays to detect any stones he may have.
If he stops urinating tonight then you will need to see an emergency vet tonight as a blocked cat can become very ill and die from high toxic levels.
If you have any more questions, please feel free to ask.

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Always consult the advice of your veterinarian if you are in doubt.
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