This social story doesn't cover that particular problem, but you could easily adapt the text:
http://www.polyxo.com/socialstories/ss0023.html
Good luck!
Here's someone who used music to get their kid on the field and participating.
http://fragilex.wordpress.com/2007/06/08/never-underestimate -the-power-of-music/
My ds plays soccer Tues& Thurs eve,when I take him he always says He does not want to be there,He says Its to hard,And he is nervous, so last night (Most nights),for the first 10 min he is trying to go home,he will even run from me if i try to take him onto the field.
Finally I say OK lets go,But thats it for soccer,we will not come back!!,he starts to cry,he says I need Help, and He comes with me out on the field ,I have to stand on the field, and back away slowly. another 10 min.
This happens every night, The thing is once I'm gone (Sitting on the side),he does great!!, in fact he is one of the best players ,Once he is in his comfort zone he is very focused ,and FAST.
He is on A team with 5-6 year olds,the kids don't seam to notice my struggles to get him on the field,But the other parents sure do!!
Any Ideas on how to help him.I would like to fix this this year because next year he will be with the 6-7 kids and they will start to notice.
Thanks,Linda