I have a 13 year old shitzu and he needs his teeth cleaned. I don't know if he should have to procedure or not?

We had his blood work done and everything was healthy. For being 13, he has a lot of energy, but he has already been put under twice when he was 7 for a surgery when he broke his leg. He is also blind now from an infection that triggered glucoma. He is a tough little guy, but I am worried about putting him under at his age. I am not prepared if he doesn't come out of it, but I am also afraid that if his teeth are not cleaned, all that bacteria will go into his blood steam and attack his healthy organs. We also think that he has Cushion's Disease. I costs a lot just to diagnose it, but he has all the symptoms and even the vet said that all they do for that is just maintain it by giving him pills and I don't want to do that. It isn't fatal and we are doing supplements and anything natural for him instead. With the condition and being old, he is more prone to infections so that is another reason why I am not sure if I should have his teeth cleaned especially if his immune system is a little weaker. He has a lot of tarter build up and his gums are not good. Is there anything I can do to help his teeth or gums in that stage? I suspect that his teeth are sore. He swallows most of his food so I am guessing that some teeth need to be pulled. We have been giving him soft food on a regular basis but still give him hard food because he will eat and I think it will still help his teeth. My husband and I have been undecided about this for over a year and I can't say that we will have the procedure done and not be okay with something happens to him. What are some choices that I have and is there anything I can do for my baby?