I have no personal experience with his age as my child is younger, but it seems to me the reasons you began looking for another school have not changed. I would ignore the adults at your son's old school, and focus on your son's experience of school and whether he likes it or appears to be gaining something from it, or not. You and your son know your son best.
Personally I'd ask him to try the new one for some predetermined amount of time and if that one isn't working either for whatever reason (could be some as yet undetermined completely different issue) not make him miserable by insisting he continue, unless he is super shy and you don't trust his judgement about it. As you said pre-k is not compulsory and your need is really just social exposure.
There are lots of ways to get social experiences where he has to fend for himself amongst a group of peers, you could have him attend summer daycamps next summer, classes, etc. (And for non school things you are allowed if you choose to fudge about the birthday too so that he can be with slightly older kids, a possibility I learned about on this board and fully intend to take advantage of next summer so my DS can attend more interesting camps than he might otherwise).
Polly
