Awesome, that's fantastic, wait its not awesome that your in the no quad club now but theirs nothing like family time together.
We're trying to get our son into ATV's as well, I don't think its ever come up but our son has aspergers. While I don't know how severe your sons autism is I know that any form can be very trying and greatly rewarding when there is a break through in behavior but no matter what they are our children.
Keep us updated on the buggy build.
You'll sure be missed this year. If my sister doesn't go I'd be happy to bring an extra bike for you to ride.