Nothing was near him. Only our other cat was sniffing him. His eyes was wide open and dilated. Bit confused as to what cat we're talking about now

The one that died, his eyes was wide open and dilated. Our other cat, yesterday his eyes wasnt dilated when he acted scared.
This is a pic of my cat which died ;< Just under 18months old when i took this pic
We may get another kitten. We got 2 cats so they could keep each other company, isnt very nice outside so didnt want them to go outside. Our cat is very 'timid/nervous' Would he get along with a new kitten?