I agree - rule out a biological cause. Any chance the child has Prader Willi syndrome?
http://www.nlm.nih.gov/medlineplus/p...isyndrome.html
However, FERPA will not allow you to speak to the health provider without a signed release from the parent. However, for any PDD child, you / special ed should already have a release good for a year to talk to physican. If the child is on meds that you are adminsitering, some states require that you be able to speak to prescriber and the pretense here would be that the dosage may need to be adjusted due to extreme weight gain.
Have you conducted a home visit yet? You might get a better handle on what the real issue is. Don't go alone - go with someone from either the special ed team, the teacher or principal.
What is your schools wellness policy prescribing for all kids and staff , including special ed in terms of activity and nutrition education? Can you use to highlight issues for the entire class?