I am truly sickened that your son's preschool would do this. Please keep in mind that not all preschools are alike! I was reading some of the other responses to your earlier post and agree that the whole point of preschool is to introduce the idea of a more structured environment for children of all abilities. Some kids have never been in that kind of environment and need a little extra coaching, that's all! Unfortunately, many public schools have expectations that are one-size-fits-all. The teachers have certain goals that have to be met and they need all the kids on the same page quickly. And that's where kids who need a little more initial coaching can fall through the cracks.
My child is private-schooled at my local Lutheran church/school and they offer preschool classes down to 3 years old. They are only 1/2 day classes 2, 3, or 4 days a week depending on what the parent wants to enroll their child in. Then Kindergarten is 1/2-days, 5 days a week. They gradually get the kids into it. They don't just expect a kid to be able to sit quietly and share, etc, when that's not something they're accustomed to doing. This school nurtures the explorative nature while beginning to reign the kids in to get them used to what they'll be getting in Kindergarten. Perhaps this more nurturing environment would work for your son? I know it costs more, but I've found it to be incredibly worth it for my also-curious, now second grade son.
That all being said, I wholeheartedly agree that you need to find another preschool. If you're currently using the public school system, yes, this probably means having to shell out some bucks for something private. Even enrolling him part-time in a daycare might be an awesome experience for him! He'll get all the "education" of a preschool (A-B-Cs and 1-2-3s) with the socialization skills of a daycare. And preschool is NOT required, so it's up to you how much or how little you want him in that environment. Childcare settings are significantly more flexible when it comes to this. Couple days a week, couple hours a day could work wonders.
Best of luck to you on this!!