Hi, fellow Canadian here,
I held my Dec 21st boy back. I didn't put him in JK, sent him to SK when he should have been entering grade one.
So many people said that I couldn't/shouldn't hold him back. But I knew that my son needed more time.
The principal was thrown for a loop, but I was calm in my dealings because I knew that he was too young. Not just because he was a boy, but because of his temperament (very very shy, emotionally immature). I knew that the school wanted my son at their school (as they are always dealing with low enrolment and the possibility of having the school closed over the low numbers) and I knew that I could homeschool him if they insisted he start in grade one.
He is now in grade 6 and I do NOT regret my decision at all. He does well in school, he has to work hard but he does not struggle as I knew he would if he was in his "proper" year. Socially he does well, he is still a young 12 year old and when I see the differences physically in the 13 year old boys in his supposed grade I am so glad he is in the younger grade.
That said, I have seen other boys who are end of year babies, and they do just fine in their year.
Red shirting (which I had never heard of prior to mothering.com discussions) seems to be when you want to give your child a leg up. There's a huge difference between not wanting your child to suffer through school vs be the best in their class.
Follow your instincts, you know your child the best, he might be just fine with extra support, or he may need to wait another year till he is ready. Please do not listen to those who insist you are not able to do what is best for your child.
Best of luck with your decision,