My guess is that this is stress-related. I'm wondering if the reports of joint pain could have started with you asking him about it specifically when he was first seeming ill, because you were thinking about the MMR? If so, then he might have said "yes," and continued to report it as a symptom, just as a way to indicate how bad he is feeling. I'm not saying you shouldn't get him checked, I'm just saying that all the symptoms could fit a stress explanation.
On the stress scenario, it seems like the worry is, is this an extreme reaction to something relatively ordinary going on at preschool? Or is something really really bad happening to him at preschool? Neither one is a happy thought.
You don't say whether you're planning to pull him immediately or at the end of the school year. Is there any way you can pull him immediately, or at least keep him out for the next few days?
