Compare how intelligent a crow or parrot is to its brain size.
Size is certainly important, but it isn't everything, a bird's brain is simply more efficient than a mammal's.
Given this kid's alertness and emerging verbal skills, perhaps his is also simply wired more efficiently?
Not to compare the extremes, but to make a point, I had a head injury seven years ago. Just last year one aspect of the damage healed. That was SIX years later. My typing went from 16wpm for a number of years back to almost 100wpm, like before.
The brain is amazing and I hope neuroplasticity helps this child have a great and wonderful life, realizing the seriousness of his illness.
Plus he is only a year+ old, and technology is moving at lightning speed. Chip implants to help him along? maybe, in the future?