Mm, samzaffar123, can I suggest you handle DS's withdrawal from this nursery cautiously?
They sound like they combine unprofessionalism with high-handedness with quite a patronising attitude towards you (expecting they don't have to tell you anything, but you're supposed to go off and write a diary for them).
I'm also getting the slight itchies round them behaving like this to the only mixed race child in the nursery.
It may be nothing. And certainly DON'T PANIC!
But I would just plot a slightly defensive route for this withdrawal.
When you're ready, state your intention to leave IN WRITING. Keep a copy. Outline the events above. Thank them for their concern for your DS, say that you are extremely disappointed that they got as far as three external assessments for your DS without even mentioning to you that he was mute at nursery, never mind asking you how he was at home or giving you the opportunity to work together on any needs DS might have. State that it is their lack of communication that is the reason for you changing DS's nursery, as you are not confident they are behaving in a way that is best for DS. (If you like, cite the toileting issue as a way your DS has been disadvantaged by their failure to tell you what was really going on so you could effectively work together.)
Phrase it in terms of best care for DS, not in terms of "how dare you do things behind my back".
This is probably overkill, but it won't hurt to do it.
Because if the nursery takes umbrage at you having challenged them (in the mildest way) yesterday, they could cast you as an unco-operative parent removing your child from the caring professionals who are doing their best to help him.