All I know is that when both of mine started school, they had an already well-established vocabulary which they could use to easily communicate with the adults in charge of them
I baby-talked both of mine. The eldest has been noted to be exceptionally articulate, his nursery teacher told me on several occasions that she often forgot she was speaking to a 3/4 year old. Maybe your ex was the reason your son could communicate so well?
I can't imagine being a teacher, or TA, in a class of 30 Reception class pupils, who each call a toilet by a different name
How many words do you think there are for toilet
. Very likely a reception child will know a toilet is a toilet. Less likely a potty, or similar depending on local dialect that a teacher would be well aware of.
(my son's younger siblings, for instance, still - at 11 and 9 - call it "a wee-poo"), and having to work out what they want.
Your son's younger siblings are very very much in the minority. That's not in anyway typical or an argument against using baby-talk.
There's a lot of linguistic snobbery on this thread. People thinking that being ever so well spoken will result in more articulate children. They all start talking rubbish when they get to their teens, regardless of whether they spoke 'properly' beforehand or not, so it really doesn't matter in the end.