Puppies learn to inhibit bites when they play with other dogs, and owners also can teach limits. (petsmagazine.com.sg)
I am wondering if perhaps your tone inhibits him a bit? (pethelpful.com)
Interestingly, puppies learn to inhibit their bite when they are bitten by another pup, and at this stage, they start to understand canine language. (thesprucepets.com)