I'm going to move this from the disABILITIES Community Board to the disABILITIES Board. I think more people with WDW experience will see it there.
You might want to start by looking at the disABILITIES FAQs thread; it's near the top of the disABILITIES Board or you can follow the link in my signature to get there.
In order to help, it would be helpful for us to know a little bit about your foster brother. Here's a few questions to start out:
I'm assuming that DD = Develpmental Disability
FAS = Fetal Alcohol Syndrome
Does he use a wheelchair? If so, what type (power or manual; tilt in space)? Can he get out of the wheelchair to board rides?
If he can walk, would he be able to walk 3-9 miles a day?
How much assistance will he need for getting on/off rides?
Is he able to talk?
What is his level of understanding?
Does he have any hearing or vision disabilities?
Does he have any issues with behavior?
Are there any special things he likes?
Any special things he dislikes or is afraid of?
Does he have any restrictions that would prevent him from going on any attractions?
Has he ever been anywhere at all similar to WDW (like a local amusement park)?