I wrote a whole bunch yesterday, but I erased it, because I thought it was too much about me and not about helping you. I'll start again:
The vet said we can avoid the risk of surgery as the teeth will rott fall out eventually
This disturbed me a lot.
BUT one of the teeth likely causes him some pain.
My thoughts are: Then do what you can, now. He's not getting younger.
I do not consider 12 to be old, but maybe middle aged.
I kept asking my regular vet about my kitty Ginger, with kidney disease, about a dental, and she kept putting it off. Last year Ginger was off in her eating, and I asked again. The vet finally agreed and she just had a dental last August, when she was 16. I just made sure to read all that I could on kidney disease and dentals and to relay what I wanted.. to the vet.
My kitty J.D., who was diabetic for his last 9 years (and lived to be 20) and the reason why I came to this forum, had his first dental at 14. I had to ask my regular vet about a dental for him regularly, and he finally (after years) agreed. The vet kept saying something about his diabetes not being controlled. Well, it hasn't been controlled for years. And my thoughts were, he's not getting any younger. I was worried, because what if my vet was right and something went wrong and it would all be my fault. When I went to pick him up after his dental, I asked the vet tech that was handing him to me, "How many teeth did he have removed?" She replied "Oh, I don't think he has any teeth left."

He had 8 teeth removed.
When J.D. was 18, when asked repeatedly about another dental, my regular vet kept saying "No, not at this time". I felt like he wasn't eating right, so I took him to a board certified veterinary dental specialist, one that does x-rays before and after the procedure (and was well known for his expertise in the field) and I had J.D. go through a cardiologist appointment and an echocardiogram first to make sure he was okay for the anesthesia. I also talked to the anesthesiologist beforehand to make sure she knew about his kidney disease, diabetes, age, and what not. She helped to relieve my worries. During his procedure, this wonderful dentist/doctor sent someone out to where I was waiting outside in my car to tell me that J.D. had a majorly infected canine, and to ask my permission to remove it. Of course! Upon discharge, he showed me the before and after pictures of J.D.'s mouth (photos and x-rays and tried to explain everything to me), and he gave me the canine tooth in a little vial. It was almost an inch long (so, maybe 1/3 is below the gums, and 2/3s is above). J.D. had to have strong antibiotics and pain meds afterwards, but I was happy to help my J.D. not have to have that pain anymore. My only next worry was telling my regular vet what I had done.