Specifically, he can easily read Dolch sight words when he sees them in isolation (he often misreads or skips them when reading sentences), but he is horrible at spelling them. He still has trouble spelling words like ‘was’ and ‘what’ and only starting spelling ‘the’ with confidence a couple of months ago. He also has very poor handwriting and he often asks me things like “How do I make a ‘t’ again?” How can a seven-year-old with an FSIQ over 150 not know how to make a ‘t’ ???
I can't answer your question specifically about ability to visualize, but the above description of symptoms including very poor spelling and extreme handwriting struggles sounds like dysgraphia. Your DS could very well have FSIQ over 150 and have dysgraphia. Has he had any achievement testing or other diagnostic testing? If you suspect dysgraphia, and I certainly would, there are additional tests that will help rule it in or out.
Did you discuss his struggles with the psychologist? What did she say?