Sounds like gestalt language processing. My son was exactly the same and hes 7 and still not potty trained. I have never been able to get him to sit on a potty or a toilet. No professionals would believe me until they also tried. Continence team have been no help to us, and a woeful lack of children's OT in my area.
Please look up gestalt language processing, it's common among autistic children who are pre-conversational. It's been a few years since we had SALT but it was only during our last sessions that they were actually trained on GLP, so it may not actually be rolled out with NHS everywhere in the country yet.
In my sons case he is now "verbal" but you won't get a conversation out of him. He can say a lot, and he forms his own sentences, but if you ask a question he can't tell you an answer. He sounds fantastic when he's in his own little world but the most you get from him directly is about what he wants or rather doesn't want.
Also, please don't fret about school. A. If you send him to mainstream they will support with intimate care, and can provide invaluable evidence for EHCPs. B. You also don't have to wait for school to start the EHCP process if you think you might need to go straight to specialist, you can apply online yourself on your LAs gov page.
You also don't need an EHCP to ask specialist schools if you can look round, ask questions, find out how they support children with communication issues etc.
You don't need a diagnosis for an EHCP either.
It is scary, but take it from many of us who are years into this, it gets less scary, and knowledge is power. You are your child's best advocate, nobody knows your child like you do, and you know more than you think you know and are more capable than you think you are.