I don't think he would have been invited. There were not many celebrities there anyway. Besides, prince William doesn't have any ties with Michael. Michael knew Diana but he wasn't close to Harry and William. I think the Beckhams were invited because William is the president of the English Football Association and they know each other after leading England's failed bid to host the football World Cup.
Anyway, if he had been invited I am sure Michael would have worn a smart dark tuxedo, nothing too flashy.