I will answer the last part first.
Usually you will not find CMs hanging around near the restorooms. Even if you found one who agreed to keep an eye out for your son, the CM could easily be distracted by another guest asking questions and not pay attention to watching for him, or another CM could come up and tell that person it is time for them to leave on a break, or anything which would for the CM, take priority.
When you say he is autistic, the question comes to mind of what are the specific problems/needs. How does he handle going to the bathroom in school, or in other public places? Is he able to follow instructions? For example, can you wait until he comes out and then tell him to sit or stand in a particular place until you return and then you go to the bathroom?
Don't forget, that for some people the companion restrooms are the only ones they are able to use.