I think the reaction from the school about reading is a wonderful start. If they bristled, or were otherwise dismissive you might be off to a bad start (though at 9 hours per week your son could still benefit from at least trying to have the experience of hanging with his peeps).
I'm all for giving the child a chance, as well as the school, if the signs are good at first. 9 hours is probably 3 days for 3 hours each? That would give you plenty of time to keep doing the things you've been doing with him so far.
If you live in a community where the majority of children start a preschool program at least by the time they are 3, and you're planning to send him to school by the time he gets to kindergarten age, it's good to at least try to have him start when his peers start. Make sure you understand the refund policy in case you end up feeling like you need to take him out in the future.
I'm always amazed at how many parents with very, very young children have already found their way to this site and are so organized and knowledgeable about their child's skill base! If I was that organized and proactive maybe my DD would have had a less rocky time up to this point.
