My advice is probably contrary to what you're asking for, but I strongly recommend following the advice of the teaching professional (the Kindergarten teacher who has observed him all year) and hold him back. Sounds like he's young anyway, and being young will continue to plague him throughout his life, even into college, and then job interviews when he graduates. In Jr High and High School, he will be younger and therefore less mature to deal with all the challenges and peer pressures kids encounter. (drugs, sex, social situations, etc.)
Academically, everything is measured by grade, not by age, so being young is adding an extra challenge for him to compete. You don't want his self-esteem to be affected because he feels like he's behind his peers. They do notice these things more than we know.
My sixth grader turned 6 years in August of his Kindergarten year, and we've NEVER regretted it once. We talked with dozens of educators, and they ALL recommended holding him. He's always been an honors student, but we'd rather work to keep him challenged, than be scraping to have him keep up and trying to repair his self-esteem in the process. My other son has a March birthday and I notice that's it's harder for him.
The other thing about boys is the "playground" thing where you never want your child to be the one that other kids think is weak. Another year will give him more size and more maturity. The schools do a great job, but it's just an inherent social thing with boys to compete, and it gets more intense with age. And, the girls are sooo much more mature than boys, so it's good to have more maturity for social situations starting in 4th grade, Jr and Sr High.
With all-day K next year, I'm told it will still be offered, but at a cost. But apparently it's a reasonable cost comparatively, and I think it'd be a good investment for his future.
I offer this advice from my heart and from my love of kids and seeing them be happy and succeed. Good luck!