The research I have found supports holding back kids with a summer birthday and indicates test scores, math achievement and reading outcomes are significantly better for summer birthday kids that did not start kindergarten until age 6. I also found 1 article that indicated that gifted kids with a summer birthday often go unidentified due to being overlooked by teachers because they are young.
Actually, the newest research seems to show the opposite of what your husband believes.
http://www.newyorker.com/tech/elements/youngest-kid-smartest-kidThere's also a school of thought that suggests children who are unchallenged in elementary school lose important opportunities to develop executive function skills.
http://nurturingwisdom.com/is-executive-functioning-the-missing-link-for-many-gifted-students/Anecdotally, my twins are June birthday boys with a September cutoff. They were not redshirted and there was only one other boy younger than them in their elementary school--the rest were redshirted, so it's very common in our area.
My observation of the redshirted boys in their grade were that they were uncomfortable because they were much bigger than other kids, they weren't any more advanced, and I know from one family who red shirted their kids-- the children felt dumb because they had been held back.
In one case, the kids were going to be tall anyway, but when they were redshirted, they were literally more than a foot taller than their classmates by 3rd grade and were at least a year older. They might have been awkward no matter their grade, but they had a hard time fitting in with grade peers who were younger and much smaller. But the additional issue was that they weren't doing any better academically, so it wasn't as if redshirting gave them an academic edge.
