If he even has one, what is Dragon's devil fruit power? What is Dragon's devil fruit power? 

In Loguetown, it became stormy out of nowhere when he showed up. Also, I think I remember Smoker saying something about it.
The only accurate answer is we do not know.  There is no proof Monkey D. Dragon has a devil fruit or that he does not have one.

There are many theories attributing the lightning strike that saves Luffy from the gallows to Dragon and, therefore, lightning to his devil fruit.  They hypothesize that he could have the storm storm fruit or air air fruit.  These are all speculation, however.