My son has the same issues. Before they swapped over to signing up. he stood in line for the whole process and then faded in to the background when they got to him he was so overwhelmed. we watched the show and then moved onto the rides.. towards the end of the day he asked if he could try again.. so we went back and stood in line and he got chosen and he had a blast..
Sign your son up for the last show of the day,, get him to watch a couple of shows. if thats not possible , then get him to watch
Youtube videos of the show.. and talk to him about what is happening.
and if by the end of the day , he still cant do it, then he hasnt spent the whole day upset that he couldnt do it ..
You can stand near at the circle. at either end , which will be easier for him to eye you.. make sure he knows the spot you will be standing at!
Hope he can get to do it.. but if not , thats ok too.. there are a lot of kids that cant for whatever reason or dont want too..