Susan Mary, you have my sympathies right now. I do not envy you this decision. I cannot read between the lines, but I feel your vet is telling you that you may well lose your boy if you go with surgery. Even if you do the surgery your time left with him will be very limited. Is the risk of the surgery and the subsequent recovery time going to be worth it in the end to your boy Max?
I too have a senior dog (Shadow and Akita X). Her health is failing. I know that I will not put her through surgery to prolong her life for my benefit. She has had a good life and when her time comes I will send her to the bridge with dignity and all the love in my heart and soul. I know I will never be prepared for that day, but I know in my heart that I will do what I have to do for her sake. I owe her this much.
I agree that it's hard when people say "you will know when it's time" but I think you will and do know the answer to that. If your boy only wants to be with you, but already he's not allowed on the carpets how hard is that on him? In these his final days he does not deserve to be separated from you for something he cannot prevent. Bless his heart.
I wish you the best during these difficult times. Know that we all know what you are going through and are here to offer you any help and support that we can. Be strong for yourself and for Max. He has given you his life and now it's your time to help him.