Puffin has good advice. Is it still possible to homeschool him for now? What is your relationship with the school administration like? Given that you have older children who went there, maybe it is something you can bring up informally to see what they think the options are in his case to meet his needs given his cognitive test results but lower achievement scores.
Please don't take this the wrong way, but it seems you are blaming yourself and perhaps panicking a bit. It may be prudent to stop for a bit to let this all sink in and let your brain think of other options for now.
