Print this and show it to the doctor who denied him. this is copied from MO state statutes
10. Unless a parent has been denied custody rights pursuant to this section or visitation rights under section 452.400, both parents shall have access to records and information pertaining to a minor child, including, but not limited to, medical, dental, and school records. If the parent without custody has been granted restricted or supervised visitation because the court has found that the parent with custody or the child has been the victim of domestic violence, as defined in section 455.200, RSMo, by the parent without custody, the court may order that the reports and records made available pursuant to this subsection not include the address of the parent with custody or the child. Unless a parent has been denied custody rights pursuant to this section or visitation rights under section 452.400, any judgment of dissolution or other applicable court order shall specifically allow both parents access to such records and reports.
11. Except as otherwise precluded by state or federal law, if any individual, professional, public or private institution or organization denies access or fails to provide or disclose any and all records and information, including, but not limited to, past and present dental, medical and school records pertaining to a minor child, to either parent upon the written request of such parent, the court shall, upon its finding that the individual, professional, public or private institution or organization denied such request without good cause, order that party to comply immediately with such request and to pay to the prevailing party all costs incurred, including, but not limited to, attorney's fees and court costs associated with obtaining the requested information.
SPARC also has a request letter to send to doctors and schools who think they have a right to deny a parent access to their childrens records.