I don't often read the Primary Ed board as my DC are older, but from the POV of someone who has this behind them I thought I'd weigh in.
My dd2 was born 27th August and started school full time, no flexibilty, eight days after her fourth birthday, and went into a class of 30, which was about 75% autumn born.
She definitely wasn't ready, she could just about recognise her name (four letters) , had no clue about phonics, not independent in the toilet, had speech delay but no Statement, and struggled with making friends. I was heavily pregnant with DD3 as well- just to throw that into the emotional maelstrom.
I won't lie and say it was a breeze. If I'm honest all of reception and Y1 was a bit of a struggle. She would always rather be playing, climbing, getting messy etc, and just didn't understand the point of school, she was too busy for it!
But it fell into place at the start of Y2.
She is 15 now and the youngest in her Year to sit her GCSEs this year.
She is predicted A*/A in all her subjects (though I have to admit, her teachers still do pull their hair out a bit with her disorganisation!)
He will get there, be ready for plenty of love and cuddles when he gets home.