The doctor just came in and released Karl to go home. Apparently the fluid that they saw on the CAT scan this morning is actually just a combination of blood, fluid, etc from the surgery. It is located outside the skull, beneath the skin. This is a good location for the fluid because it is outside the brain. The fluid is probably just the result of two brain surgeries in about 2.5 weeks. The doctor said that the MRI and CAT scan look great, and that Karl's brain is healthy and is starting to expand and fill the void left by the tumor. So yes, Karl's braining is growing which of course means he's getting smarter.
We have to monitor the fluid and swelling on his head but the doctor anticipates that this will decrease throughout the next few days. If it doesn't he may need to get it drained but this appears to be unlikely.