Everything I've read says fanning is a natural thing for male turtles to do. As long as it goes back in by itself everything is fine. I am concluding from what I've read about this it will occur when ever he feels like it and he'll continue doing this when he wants to.
But Steve has a great point, if there is gravel in his tank smaller than his head he could have ate that and become impacted, requiring surgery if the stones don't pass.
What size is your turtle (SCL: shell from tip to tip not including curve) and how old is he?