How does one go about starting a trust fund for a developmentaly challenged son.

Some reading for you:
How to Set Up a Trust | eHow.com (http://www.ehow.com/how_2820_set-trust.html)
Pulmicort - FamilyEducation.com (http://life.familyeducation.com/trust-funds/money-and-kids/48035.html)

This one is close to your request but it's based in the UK:
Setting up a trust for a disabled family member (http://www.sense.org.uk/publications/allpubs/family/F03.htm)