Hi Belana, it looks like his processing speed has been most drastically impacted, and his working memory. I've just finished reading a book mentioned in another current discussion here "bright kids who can't keep up", I have mixed feelings about it but you may well find it very useful.
Chronic illness certainly impacts cognitive function, I have a chronic degenerative condition that i know has an ongoing impact on me. I imagine focusing on his health is the best way to help him? And putting in place the sort of supports mentioned in the book I suggested.
