Dogs and puppy mouthing the neck?

I have a 15 week old lab mix and a 9 year old shepard mix. The lab has been in the house since he was 6 weeks old. They have started playing more aggressive, but my 9 year old dog loves playing with the puppu. However, the 9 year old will flip the puppy on his back and put her mouth over the puppies neck. The puppy loves it but I'm afraid of him getting hurt.