Welcome Green
You'll find lots of great advice here!
FWIW, my ds12 (7th grade) had a birthday 2 months past the kindergarten cut-off. We desperately wanted to send him to kindy the year before he was eligible because intellectually he was beyond ready - but we couldn't. Our school district is so extremely rigid about the birthdate cutoff that they don't make exceptions for new incoming students until 2nd grade, so even our local private schools follow the same rule re age starting kindergarten.
That's part 1 of our ds' story - part 2 is, he's very short. So am I. He's not terribly athletic - in fact, he's generally less coordinated than most kids his age (he has Developmental Coordination Disorder which impacts how well he can catch a ball, he runs very slow, can't figure out things like skipping, and he was older than 5 before he learned how to ride a bike). He is extroverted by nature (prefers to be with people) but in early elementary he was extremely shy around adults and not terribly outgoing with same-age children. He's also a bit sensitive about how he perceives other people see him.
Soooo.... he fits most of the reasons you're considering not sending your ds to kindergarten until next year (short stature, not great at sports, and sorta-kinda-not-outgoing). From my perspective, each of those traits are things that are innately part of who we are, not things that are going to go away with age. Some things will change - as kids mature they often find more self-confidence in making new friends or talking to people they don't know well. Children do get more coordinated as they grow and most eventually learn how to ride bikes. But a kid who is going to be short is going to be short, no matter what grade he's in - not just when he's full grown, but most likely all along the way. The variation in height in each of my children's classrooms throughout elementary school has been huge; my older dd is relatively tall, but she's been with same-age peers who tower over her. My ds has always been the shortest boy in his class and sometimes the smallest kid in his class, and there was an age where this bothered him a bit (I think back around 2nd or 3rd grade) - but the thing is, he would have still been short and would have still gone through that phase where it bugged him no matter when he'd started school. AND keep in mind... we didn't have the option to start him early so he's always been one of the older kids in his class.
Would I have started him a year early if we'd had the option? For academics, absolutely yes. Have I regretted that we couldn't start him earlier? For the vast most part of everything, no. He did get bored with academics in elementary school, he probably didn't ultimately advance as quickly ahead-of-the-game as he could have had he started one year earlier, but we after-schooled a wee bit in the areas he was most interested in, and at the end of elementary he was able to use his after-school work to advocate for subject acceleration and we found middle school to be a place where subject acceleration has worked to give him an opportunity to have academic challenge yet still be with kids in his age group, which is a good fit - for him. It might not work as well for a different child with a different personality - but sometimes there's no way of knowing until you get where you're headed to, kwim?
If I were in your place, I'd put aside worries about height, sports etc and trust whatever my inner gut feeling is telling me. You know your son best
The last thing I'll mention - for many of us with EG kids, school just isn't ever going to be a great or perfect fit. Although I mentioned that middle school has been a better fit for my ds in terms of academics, he's still after-schooling in science so that he can learn at college level in an area of academics that he loves. He's matched ok with school and language arts - that's not his area of strength. He's ok with his math placement at school (he's subject accelerated a few years). The pace of his other classes (history etc) can be a bit boring at times (even the math) but he's ok with that. High school holds out promise for more challenge with the classes that he'll be able to take - so in many ways, it would have been nice to be one year closer to high school at this point, but on the other hand, it's also been ok. Not sure that makes sense, but what I'm trying to say is - it's not going to be easy (most likely) no matter when your child starts school, but it's also going to be ok.
Best wishes,
polarbear
