I'm not a diagnostician... but it seems like an incomplete picture of things. Tossing the details together, it reads more of a narrow challenge area.

Is he able to tell stories? Recount things that happened to him with semantic details? Does he build things or do art with original content? Is he a bit of or even extremely perfectionistic? Does he take a long time to make decisions when things are of similar value to him?

I noticed the difference between word generation semantic and letter. He may just have too much going on in his head and needs help narrowing down his thinking. One of the cooler concepts/tools out there for creating creativity constraints or ideas to spring from (works either end of creative blocking issue) is having a set of pictures to work from... such as these from IDEO:
http://www.amazon.com/IDEO-Method-Cards-Inspire-Design/dp/0954413210

When stuck, you randomly grab a card, e.g. it's a steering wheel. Then ask yourself "How can I write this/answer the question in a way the makes me think of steering wheel?" or such.