I am so glad one baby is OK, that's GREAT. Here is how to keep him healthy.
First, do not put any flea poison on him because it can make him sick.
Get a bag of food grade diatomaceous earth on ebay or amazon.
Put it all around the perimeter of your house, under all sofa cushions and mattresses. It will naturally kill all fleas and s safe for cats.
Get a few cans of pate cat food. Add a tablespoon of warm water to make it soupy. Feed him small amounts every two hours as that is the best way for his body to metabolize calories.
If he gets dehydrated give him tuna water drained from the can.
Please let me know how he is doing .
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