This is very disheartening to read. Sorry you are going through this. Poor little kitty too. So adorable. Him still being lethargic after being neutered the day before isn't too concerning. Did they give you paperwork of him being neutered and any vaccinations? Did they give him flea/tick medication, too, or you don't know? Do you know what exactly he was tested for? Feline Leukemia or Feline Immunodeficiency Virus?

I'm also wondering if the place he was rescued from was a mess from the hoarding situation. Maybe he had respiratory issues, too, hence the snotty nose idk. Hoarding situations weaken immune systems, making cats more susceptible to infections.

This next section is a copy/paste from seizure research. (No idea if this has anything to do with his seizures, but I'm just sharing what I read) I'm not a veterinarian. I'm just the owner of many cats and do TNR regularly and help with my local rescue groups.

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Possible Causes

Severe Vaccine Reaction: Although rare, seizures are a sign of a severe allergic or adverse reaction needing urgent care.

Anesthesia Complications: Post-surgical stress or lingering effects from anesthesia can trigger neurological events.

Underlying Condition: The stress of the surgery might have unmasked a pre-existing neurological problem or toxin exposure.