The reason for the PTSD may be important to think about for certain attractions (no one is expecting you to share that information), but in looking at attractions, you should look at what the attraction might include that old be a problem.
For example, one person I know of had PTSD related to working as a firefighter at the World Trade Center on 9/11
She was not expecting or prepared for scenes of the 9/11 site in The American Adventure in Epcot, which was very distressing to her.
An earlier poster suggested watching
YouTube video, including the line.
In post 3 of the disABILITIES thread, there is a link o an interactive YouTube video tour of WDW. It could be very helpful because it actually does show at least part of the line and the boarding for each ride.
Since you have small children, concentrate on attractions that don't have height restrictions, since one or 2 of your children would be too short for them.
The most helpful 3 pieces of advice I can give would be:
1) You wrote that a day at the zoo can be very stressful to him.
What makes a day at the zoo very stressful?
My experiences at zoos haven't involved enclosed spaces full of people or invasion of personal space.
You may know those things are difficult for him from other situations, but there must be other things besides those that make a day at the zoo stressful.
So, think about what those thing might be because chances are that you will run into those same things at WDW.
2) What coping mechanisms or tools does he normally use to handle stress or help with his PTSD? How can he use those at WDW?
If he is working with a therapist, can the therapist help him to come up with some strategies?
3) Use a touring plan like
www.touringplans.com or
www.easywdw.com
Even if you don't follow the touring plan exactly, but knowing which park is the 'recommended' park for the day will help in ways that a Guest Assistance Card can't. GACs can only help in attractions, and may not actually be all that helpful for anxiety type issues.
Knowing which park and which part of that park to go to can help avoid crowds all over, in and out of attractions.