Aww really sorry to hear Franta took a turn for the wors, bless him.
Try not to beat yourself up as you have done nothing wrong and have done everything that a good caring meowmy would do.
It's absolutely awful when something like this happens and it's only natural to doubt yourself, your reaction etc. He is now getting the treatment from the vets and hopefully he'll make a full recovery and be back to annoy you sooner than you think.
Meanwhile you need to get some rest if you can and don't forget your other furry ones are there to cuddle you (if they like cuddles lol).
Lots of urine will hopefully mean no more dehydration... if he's still on a drip then I'd expect that lots of urine is a good sign
Sending you lots of love and hugs, Kerri, Davey and Tinks xx