I am so sorry
I would definitely have him under a neurologist care. It could be coincidence or could be the flea med if you noticed it right after flea med I would trust your gut.
Was the ALT high as that measures toxicity in the liver which can trigger neurological symptoms.
If blood work was perfectly normal then it could be neurological.
I am just so sad reading this as can only imagine how you are feeling
If toxicity sometimes plasma transfusions can help as it rids the body of toxins faster and it runs about $290 for a plasma transfusion so you can ask about this but if blood work is normal not sure if this will help.
was this flea med given with any other drugs or flea meds as another dog on here sunny had a horrible reaction to flea meds that were mixed

Seems to be similar to me to what you are describing.
Sometimes when they sedate for seizures as happened to my friends dog that had ivermectin toxicity they have to suction the trachea which can cause fluid to get in lungs thus pneumonia sets in