Not knowing what to say or write can be a language disorder, a social skills disorder, or both. Knowing exactly what skills are missing is vital because it tells you what to work on. I'd say to pursue the testing.
ITA. I'll also add that it can be a confusing and difficult issue to see. Our now-high-school ds has an expressive language disorder, but we didn't really notice signs of it until he was first diagnosed and accommodated for dysgraphia, as well as matured enough to be able to tell us when he was struggling with output (he started telling us about it around 4th grade / 9 years old). He was diagnosed at 10 years old with the expressive language disorder, but even that diagnosis has changed a bit over the years after working with an SLP on some of the basics, then other pieces of the challenge-puzzle become apparent. I am a huge proponent of having a neuropsychological or some type of global assessment when you suspect issues like this. The diagnosis is just one part of the eval - the important thing is you'll come out of it (hopefully) with a much better understanding of what's going on with your child, as well as recommendations for a path forward to accommodate and remediate. It's not the end of the testing phase necessarily either - for instance, if the neuropsychologist feels there may be a reading-related issue, he/she may recommend further testing by a reading specialist. The key though is, when you go to those other follow-up specialists (such as the OT), you won't be going there "in the dark" with just a guess, you'll be going to the appropriate professional for evaluation/help.
In your OP you mentioned your ds "read" a large book by skipping pages with all text, and asked if this has happened with other people here. My dd who has a reading challenge did this - especially in 3rd/4th grade when the other kids in school were starting to read "big" books and she wanted to look like every other kid. The Warrior books were really popular then, and she'd check out a new book every week when her class went to the school library. She's in 6th grade now, has had years of reading tutoring and help, and *still* will say she's read a book when she's really not even looked at most of it. Reading challenges can be very tough, and if there is one, you need to figure that out early on. As the years go by in school, there's so much potentially lost in terms of vocabulary development that happens for children who aren't keeping up with reading comprehension. Plus by the time they are in middle school the ability to read well plays a key part in all their work.
Best wishes,
polarbear
