bluemagic, I can't tell you if it is or isn't an LD or other challenge for your ds, but I would wonder about the possibility of an LD based on a few things you've included in your post:
* His sister has language processing issues
* Uneven output - when he has an idea he can write really well, but other times he can't get any thoughts out
* Past evals by a psych for ASD - what prompted those evals? Sometimes expressive language challenges are a part of ASD, sometimes they are separate or associated with a different challenge. If the evals were only looking for an ASD, it's possible that something different was overlooked.
The writing process is complex - there are many different skills required to completing a writing assignment. My ds has an expressive language disorder that impacts his ability to generate thoughts for written expression. Although written expression is the largest impact of his LD, it also shows up in his verbal expression too - but it's not easy to see unless you know what to look for. DS can talk like crazy about things that are concrete and factual and that he knows a lot about - but open-ended questions and expressing feelings etc are difficult for him. We didn't realize this as early as we saw the writing difficulties simply because he was so obviously smart when he did talk - but there were clues - one of which I think you've mentioned about your ds - appearing to be shy to talk to adults. His early struggles with writing also looked a lot like perfectionism but that wasn't really the issue.
Re how to determine if there's an LD - our ds' expressive language disorder was diagnosed by a speech pathologist.
Best wishes,
polarbear
