Gifted but seems to have a writing "block"

Hi, hope I'm posting in right place, I'm new here, from overseas where resources and info poor. My 11 year old son, identified as gifted 4 years ago, struggles with writing, although does pretty well in other standard tests. Diagnosed with sensory processing issues almost a year ago, having OT including listening programme. School does not recognise there is a problem, but state-wide tests 2 years ago and recently show he has problems writing a story or description about something. A few problems with English comprehension too - can't seem to find answer in the given paragraph. What sort of tests need to be done to identify what's causing the writing "block" (he has loads of ideas but can't seem to get them out)? I'm caught between teachers (who don't see him failing because he's doing well everywhere else) and OT who doesn't know about any specialist tests. He has no IEP (no need for one as far as school concerned) and they won't refer him to school psychologist, and extension 2 hrs weekly.

Have you tried typing?

I prefer the term "freak of nature."