vickyvdw - my son has his birthday on the 10 December each year (he was a c-section baby, so in reality he might well have been a 25 December or after baby).
So I face (or have faced the same as you.)
The short answer is a child goes to Grade 1 in the year they turn 7, no matter when in the year, it can be on the 31 December of that year.
To explain my situation which might help you (or not):-
The added issue for my son is he had dreadful ENT issues until he was around 3, so that also impacted on his speech development and other issues.
When he was in Grade 0 (the one before Grade R) I realised there were some issues and I had him take speech therapy and OT to try to catch up with the other kids.
But by the time he got to Grade R I realised that I was pushing him when he was not socially and emotionally ready, and to a large degree was always 6 months behind intellectually with his class mates.
I had him re-assessed in Grade R around June and again in August, and the ST and the OT agreed that it would be better to hold him back.
The school then brought in an external/independent assessor to assess him scholastically and emotionally and she agreed to support my recommendation to hold him back.
Connor repeated Grade R.
I was emotionally bereft as for what ever reason I thought it was this huge thing and I was so unsure that what I was doing was for the best.
It turned out it was.
He was not ready.
He is much better suited to his age range now.
He excels in his work, does not fall behind, and does not have to miss playtime to catch up on work because he is struggling.
Socially is he far better suited, and is not permanently the "follower" with much bigger and older boys as the leaders.
I am really happy that I kept him behind, but I threw everything at it to ensure that I was making a good decision.
The school (he was at CBC at the time) was very supportive. They did not push, they did not suggest, they did offer me support both with helping him and when I made my decision to keep him back.
If you are unsure, first implement "assistance" - in the form of an OT assessment of chat to his teacher about his progress and how he fits in with his peers.
Every child is different, and some kids born late in the year are fine without being kept back.
<my son has a friend who was born on the 20 December and was not kept back - he is tiny in comparison to his peers, but is not struggling in any way>