Lisalisa, I recollect that you have other children but I don't know if your ds is your only or your eldest boy.
I have 4, the eldest (ds1) being 11 and a late July birthday. His school didn't use ORT but I can safely say that in yr1 he was certainly a very academically immature little boy; enjoyed books but not the most fluent reader etc., poor concentration, messy writing, you name it ! I remember that with the reading, something "clicked" sometime in year 1 and then there was no stopping him. He then progressed really quickly and was enjoying reading proper books in what seemed to be next to no time.
Your ds sounds as if he is bright and curious about the world around him and ime that's more valuable to him long term than his current reading level.
As to keeping him back, when my ds1 started the nursery attached to the school proper, we had similar worries to yours about our summer born boy, having seen just how much more "skilful" some of the other children were. But we were strongly advised to keep him in the correct peer group, and in spite of some early problems like yours (don't even start me on learning the times tables!)we are glad we did. BTW he was at a private non selective school; not really v.pressured.
I do now have a yr1 child - dd - who I have to say is a very accomplished reader (reads level 12 ORT in minutes), but we are just as laid back about her as we were with ds1, albeit for opposite reasons. I'm sure lots in her class (again a non selective school) will make that reading breakthrough soon. In fact I arrived with dd recently to find the teacher praising another little girl to her mum for doing just that!
Otoh I have a friend whose August born son, bright but immature was kept back a year, when he was about 7. He is now 10 and the senior schools cut off problem is now rearing irs ugly head for her.
I don't always believe in throwing money at these issues, but what about an ed psych assessment? Could the school do this or would you have tp pay?