This is a sad, depressing situation.
No one wants to see a 5 month old kitten go from playful to restricted movement in the span of a week.
Dasher (Kid's cat, also briefly and incorrectly termed Fatty for a few days) has IFP, a rare and fatal complication of an extremely common cat virus.
He has weeks to live, although at some point his quality of life will have deteriorated to the point that we'll need to put him down.
Probably in a week.
It is heartbreaking on so many levels.
And the fact that he and his brother sleep nose to nose isn't making it easier.