In case it is not preschool related... And I'm sure you are educated on common ailments, but just in case, Lyme Disease causes many of his symptoms. Don't know if you live in a prone area but joint pain and fever is almost always Lyme where we live. I have seen kids vomit at school before being diagnosed and there CAN be neurological symptoms from it. It is usually very treatable with three weeks of harsh antibiotics. I'm not a doc though, just live in a high Lyme area.