My ds has a September birthday and although I knew I could have deferred him (and would have done so if I'd really felt that he wasn't ready) I chose not to.
Most deferrals I've come across (I deal with registration for a local sports club, so get to know Dates of Birth and what year they are in as it impacts on which team they go into) are January/February with a couple of December and even fewer November and one October - but the earlier ones were kids arriving from abroad and using the flexibility to "catch up"/have time to adapt.
I came into the thread thinking I would say that I've only ever come across people who regretted not deferring, but as you're talking about a September born, I'm not so sure.
Ds was (is still 
) bright but was late to talk (3 
) and was also late to read (6.5 - Easter of P2, despite some 1:1 sessions with the depute head at the beginning of P2) but the school were great and said some kids (especially boys) are just not developmentally ready to read until they're 6+. He was good at numbers from the start (and memorising his books to "read" them throughout P1 
).
He ended up in the top set for both English and Maths at secondary (got an A for his AH Maths 
) and is now doing a reading heavy subject at Uni - so being late to read obviously didn't hold him back 
I think if your ds is bright, even if he's a bit hyper, school will help him learn to concentrate and focus. Being hyper might actually be made worse if he ends up being bored and anecdotally, the "bully" that ds came across at primary school (as it happens, one of his friends 
) was the December deferral as the extra age (and height?) made him think he was "the boss"