All three previous recommendations are quite correct !
It does take time for a pup to grow/learn the difference between play
and "let's stop that NOW'".
When you use a Growl to "alarm" him, don't be shy about the volume,
make it a strong one, and get his attention !
On the pinch/nipping on his neck, remember, our little guys are a LOT
tougher than they look. Make it a firm pinch and get his attention,
accompanied with a strong growl, or NO BITE !
When he played with his litermates, they growled and nipped at each
other. You just got to be a "bigger" dog than what he's used to playing
with, just GET WITH IT !
The pinches, growls and squashing him down MUST be forceful,
immediate and thoroughly convince him
you're the Pack Leader.