You could try yelping like he hurt you and stop playing with him (get up and walk away). For me though, the yelping seemed to always make him go after me more. I ended up just saying no biting firmly whenever he bit and then I would stop playing and he got the point.
Jazz, Shiloh Shepherd, 5 yrs old, CGC HIC
Bunny, GSD X, 7.5 years old