Dogs eat grass for a variety of different reasons. Sometimes they will snack on grass when they have an upset stomach. A puppy’s intent can be much harder to determine than a full grown dogs. They could just be playing, or the grass brushed up against them so he is bitting it to show the grass who is boss. If the behavior continues or you believe something may be wrong call your vet, puppies can be vulnerable to many disease so its best to monitor strange activity closely.