My granddaughter, barely 5 last year, was in an all day kindergarten and loved it. The teacher is trained and good at implementing a varied day in both activity and energy level. There was no pressure to perform. And the kids did a lot of fun activities while learning a variety of fun things. They also learned how to follow directions and to co-operate and play with others. She is in the first grade this year and also enjoys that.
Her school district does also have 1/2 kindergarten for those who choose that.
I'm guessing that the reason for all day kindergartens are two fold. One, education today is much more intense than it was just a few years ago. The full day kindergarten gives those kids an edge. And 2, most kids are in full day daycare as it is and so a full day kindergarten seems logical.
My granddaughter was anxious when she started preschool, again at the start of kindergarten, and still at the start of first grade. Just because you son objects to going doesn't mean that he shouldn't go. One of the very important things that we must learn in order to have the life that we want is how to handle new things and the stress that they involve.
Perhaps an all day kindergarten is too difficult for him. Could you start him in preschool this year? I strongly recommend finding out if he's too immature or has learning and/or social obstacles to attending kindergarten. Perhaps he has a learning disability that needs to be address before attending school. The school is required by federal law to test and provide appropriate education for children having difficulty with school.