Get him a bone to chew. That should help remove plaque. It is probably his soft food.
”# Chicken and pork-chop bones are not recommended at all. They do splinter and can get lodged in the intestines.
# Always feed fresh bones raw and avoid cooking due to splintering.
# Use common sense when it comes to the size of your dog and the size of the bone. Never give a large dog a tiny bone that he can choke on. Make sure the bone is two times larger than his mouth.
# Avoid the round steak bones completely. These can easily get lodged around the eye teeth.
# ONLY GIVE YOUR DOG A BONE WHEN YOU ARE THERE TO SUPERVISE.”
The best bones are beef or buffalo marrow bones. You can get them at places they sell up-scale dog food.