Many people here would tell you that they have an August-born children and they eventually adjust well at school.
However, all research shows that summer born children as a group have a much harder time and do less well than their peers. And now that it seems that England, finally recognised that fact and is allowing the parents to make their own decision, there is NO reason to choose a harder path for your child.
And, especially, given that he was premature, so he is not just a regular summer born...
It makes a bit difference, when a child starts school and is confident around his peers and adults. Older kids in the class are often those children. The quieter summer kids don't quite catch up, not completely. So, in my opinion, a chance (!) Of not getting a particular school is not a reason to send a child to school before he is ready.
My nephew is similar to your son. A preemie born in August. Struggled on and off through primary - both academically and socially. Now he is off to secondary and still not caught up socially. He is smaller than other children, and is still much less mature.
Dd1 is almost a summer child (end of May). She is in Y4 and is nearly top in her year academically. But if I had a do-over, I'd definetely held her back. She was/is always on the quieter side; has fewer friends; rarely volunteers for anything at would put her in front of an audience (school productions, school council); worse in sports than most of her classmates. Generally still less confident. And her August-born best friend is the same.
So - if I were you, I' look at my son and see what's best for him now. And school lottery would sort itself out somehow!