Ah yea dude, my sister is Aspergers and has huge issues with crowds too >_o also not fun when people don't see her coz she's in a wheelchair at hip height! XD The way we handle it is by sort of surrounding her (there is me, my mum, 2 sisters and 2 friends) who make a bit of a human wall to stop her from being bumped into : P and when she gets too stressed from all the people we all take a break in the corner or something! She has also reverted to barking at people in the way, and hissing at anyone who wants to hug her >.>
Kudos to you for going to the cons though, I know it can be super stressful in crowds at the best of times D:
Have you tried maybe some of the smaller cons in smaller towns/cities?