For every day, not just Disney, I would highly recommend enrolling in Medic Alert
http://www.medicalert.org/asa/ - you buy a bracelet that the child cannot remove (I know this can be challenging, but we perservered - even losing one bracelet!

) and your personal information is kept at medicalert. The great thing about medicalert is that you keep your information updated online - no need for a new bracelet if any info changes (personal or medical). You're not "advertising" a disability, but protecting your child in case he is separated from you. My son is 14 and we've been members since he was 6. Also, medical personnel are trained to look for the bracelet, which is helpful for any allergies or conditions like seizures, which my son also has.
It's a great idea to take a daily picture before heading to the park!
I personally also like "awareness" shirts - my son is autistic, so we'll wear "autism awareness" or walk-a-thon shirts. This is definitely a personal preference!! I also will try to wear a matching shirt with my son - goes hand in hand with taking that daily picture, god forbid he makes a run for it, you can easily describe what he's wearing!! HTH
p.s. edited to add that we have found the characters to be WONDERFUL when we explain the developmental delay....the best picture EVER of my son is during his first trip at age 7, one hand on his head, the other hand touching BUZZ's face and the absolute look of WONDER in his eyes (HOW can Buzz be here??)...seek out your son's favorite character!

They spend extra time and IN GENERAL others are understanding....