My experience, in the three different states in which we've lived, is that it is standard advice for teachers to suggest you redshirt boys with summer birthdays. I've heard this advice offered basically without regard to the actual child the teacher is talking about, and more in regard to their opinion that virtually all boys are socially immature at that age. But that really isn't the case, and I think the advice is way over-used. Sure there are boys that are socially immature, and arguably more boys than girls in this situation. But this is too often used as a blanket statement.
My DS11 has an end-of-July birthday and we sent him at 5 and he did great. My DS(almost 6) was ready for K last year in every way, but we knew they'd never consider early admittance for a BOY with a November birthday, so we didn't pursue it. But as time for K approached we realized at that point that he really did need to be in 1st grade. So he ended up skipping K. He's no more immature than others.
The range of kids' academic and social maturity is so vast in any classroom, that I would hesitate to question your own parental judgement. Maybe see if you can visit a K class where your son would be going to school and see for yourself what the range of emotional maturity is at the beginning of K.
