L.C.
What your child does once you leave is open to speculation --a lot of pre-schools and daycares have a child (or several) that are inconsolable for several minutes or even hours after arrival, even when they tell the parents that is not happening. They have their own reasons for lying about this, but many of them absolutely lie.
Also, when children are tested for the presence of stress hormones, it has been shown that the children who appear to be coping fine are in fact coping, not thriving --their stress hormones are as high as the children who are wailing and slamming their bodies against the doors. They just figured out that their stress isn't going to be addressed, so they don't bother to demonstrate it.
I think your son is trying to tell you that too much of his world has changed for him to feel stable without one of you two with him. This will probably last less than a year (probably a lot less than a year)...
He doesn't need preschool as much as he needs a stable life, so he can once again feel curious, explore and learn anything. No one learns anything valuable under constant stress.