Nothings been confirmed if he has or hasn't eaten a devil fruit though he is a master of haki.
He hasn't ate any of DF, the way he saved Luffy (when Luffy was a kid) is a prove. And yes he is master of haki and also a great swordsman, DF will just be a burden for him cause if he ate one he won't able to swim.
Well, swimming with just one arm isn't the easiest thing! For this reason, if i was shanks, i would have eaten a DF.