I'll chime in here:
I am a preschool teacher. In my opinion, the biggest reason for having a child enrolled is so that they can learn how to move through the day in a group and so that they can learn how to do the work of taking turns and problem-solving with other children, with adult coaching.
My four year old son is in another preschool, and precisely for this reason.
From the program you have described, I think your son isn't getting enough in the way of peers his age. Children at this age tend to start playing in groups of 3 or 4, and he doesn't have the numbers of peers to play with in this way. So, he might be getting bored. Especially if the curriculum is also appropriate for two and three year olds. He's there to learn the social piece of school (which is what his kindergarten teacher will want to see) and might not be getting enough peer socialization of the same age. And yes, you can teach all the pre-K self help/skills on your own.
If it were me, I'd look for a new program for September; one which focuses on his having a larger group of peers his age--4s and 5s. And please don't worry about the upset at drop off occuring in 4th grade or middle school-- kindergarten can be tough, but kids do come into their own about what is socially acceptable at drop-offs by the time they hit first grade. They become more aware of the perceptions of other children, so he *will* more than likely get it together.
Also, I've learned with my son *not* to ask him how his day at school is. He has discovered that having a 'not good' answer gets a lot of attention and questions from the concerned and caring adult,and boy, can we get sucked in! Now I ask him *who* he played with, or *what* he played with. This invites him to give me the facts, without his judgement or reinterpretation of them. (and fours can be dramatic) This has seemed to produce more positive retellings of his time away from me.
And kudos for not sweating the academics! I know so many parents who fret on that one. I try to tell them that it all comes together so easily in kindergarten, once we get the other foundations (social/self-help/self-care) skills down pat!