What plan is he on? This came right off of the MO HealthNet website:
How can I change my MO HealthNet managed care health plan primary care provider?
You have a right to change the primary care provider in your MO HealthNet managed care health plan. You can change at least two times each year. Some MO HealthNet managed care health plans may allow more. Children in state custody may change their primary care provider as often as necessary. To change your primary care provider or to find out more about your MO HealthNet managed care health plan, call the membership services number on your managed care health insurance card.
If all of the physicians in the area refuse to see him though... you may still have a problem. It really depends on the reason they said they wouldn't see him. It could be that they simply said that because he's a patient of another dr. Could be that your daughter gave too much info when she called. I would not mention any dissatisfaction with the other doctor when trying to change. I'd just say that their office was recommended or that it was closer to school/home/work/babysitter/whatever.
By the way... here's the website where I found that info:
http://www.dss.mo.gov/mhd/faq/pages/faqpart.htm