B.J.
I haven't read all of your responses, so forgive me if I repeat. I totally understand your concerns. I really do, as I have a very anxious child myself. That said, this is your husband's only sibling. What kind of message do you want to send to your son? In many families, a wedding of a close family member is TERRIBLY IMPORTANT!! If this is the case in your family, you should go to the wedding and take him with you. I cannot imagine my husband not attending a sibling's wedding. It is unthinkable to me.
Let's face it, Pre-K graduations are fun and cute, but how many kids don't really make it there or have to really reach deep down to achieve this goal? It just isn't the same kind of achievment as high school or college graduation, and should not be treated as such. That is not to say that it is not important, but it just isn't on the same level.
Besides, do you really feel that going through this ceremony is going to make going to Kindergarten any easier for an anxious child? It might be enjoyable--but it will not alleviate any fears/concerns about Kindergarten.
As to him participating in the wedding---have him try. If it doesn't work out, everyone will understand. While it might be a bad day for him, it truly won't cause him lifelong trauma, and you have definitely taught him a lesson about priorities and thinking of others. Good luck and have fun whatever you decide.