Stinky mouth

I'm wondering if anyone else has had this problem. Buddy is 6 years old and his mouth has been getting progressively smellier. It's not bad breath, more like stinky drool. His bed, and anything else he lays on or rubs his face against, need constant washing. We get his teeth cleaned once a year and this helps for a couple of weeks. Cleaning his teeth at home only seems to help for a couple of hours. We've also tried dental chews/bones etc. He's a great dog otherwise.

Thanks for any help/advice!