I've been fostering kittens and cat for a local rescue since July 2008 and have three of my own(failed fosters!) so I'm pretty experienced with normal and abnormal kitten behaviour. Currently, I have 4 fosters. Two are brothers, Seeley and Sully. Seeley is the picture of kittenhood. He's big for his age(about 14-16 weeks and almost 4 pounds),very very playful, loud purrer, social, etc etc.
Sully, on the other hand is extremely small(between 1.5 and 2 pounds). He came to me as a bit shy but relatively normal. Now, he is very serious, doesn't play with the other cats, mostly just lays around watching the others, and doesn't purr often. He often looks distant or 'not there'. About 3 weeks ago, he came down with a severe infection literally overnight. His bottom jaw was swollen, he was lethargic, and very dehydrated. I took him into to the rescue, where they kept him overnight to give fluids and antibiotics. They told me it was calici, although to me, it looked nothing like calici. When I picked him up the next morning, he was rambunctious and completely healed. He was purring a mile a minute, was very vocal, and wanted me to hold him 24/7. A day later, he got into my air duct and fell a good 13 feet down the duct. When we got him out after 40 minutes, he was back to being aloof and distant. I thought he might have been traumatized but he never returned to the loving behaviour. On top of that, last week he had another bout of lethargy/dehydration. I gave him Sub-q fluids and the next morning he was right as rain(just not affectionate at all).
The point of all this rambling is, has anyone else seen anything like this? I feel like whatever the first infection was may have caused brain damage. The fact that his brother is huge while Sully's growth appears delayed also suggests to me a genetic disorder or brain damage. He will be seeing a vet again for shots and I'm going to ask about his history, but I just wanted to see what your opinions were on this!