Oh, god, jimjams, same here.
iirc, your ds was the same as my dd1 as a toddler - nice and happy, content, charming, "just a bit behind" as all the professionals kept putting it.
I had one paed tell me that dd being so happy to play with a brick (one single brick, picking it up, then dropping it) for 30 mins while we spoke was a blessing as "at least you can get on with the housework"
dd1 was diagnosed with "mild delays", and so we got no help at all. She still doesn't.
It took 18 months before she got portage, a helper at pre-school, or any kind of SALT assessment (still doesn't get SALT, as she is verbal and a pre-schooler, that ticks all their boxes . she can recite the whole of the Gruffalo, but she can't tell me when she's hungry - doesn't sound like a 4 year old with a good command of the language to me)
we are now fighting to get ehr into an ASD unit rather than mainstream, as she is still "no trouble" and easily overlooked. Her last paed was at least brave enough to tell me that the issue was, and I quote, "we've got bigger problems".
well, they might, but they aren't mine, or dd1s.