Pink scar at back of mouth

I'm not a vet, but I don't see anything to be concerned about. Puppies are tough. They're built to play rough. I've had puppies bleeding in the mouth from being sliced open from the cat asserting its personal space, biting their own tongue on accident, and chewing on rawhide bones with sharp broken edges. Everybody always healed up and bounced back right away. If you found a scar on your puppy, that tells me he already healed up before you even noticed he cut himself in the first place.