Make fun of him for being different!  :spotted owl:
Whoa, a good use for speechcraft!
Well, my dad always told me that when fighting someone bigger than myself, I should kick him in the shins and run like hell!  So I guess rinse and repeat.
Game the AI and jump up on something out of reach and keep peppering the giant with arrows while it stupidly tries to hit a rock.  Might have to jump a bit higher than normal.  
Poison apple?
Reverse pickpocket a cursed item?
A sling?
Cut its hair?
OH!  I know!  send in a small person to say "oink oink" becuase if htere's one thing 
bumbles giants like, it's pork! Then push a rock on it's head from above!