I am sorry to say your little guy is getting on and with his probs he is doing well. I am afraid he is in dog yrs 98 .....now that is getting on.You say his stomach is distended sometimes
.... I would think it could be his kidneys are not one hundred % that would be just age related I would think.
Have a good talk to your Vet about him, the main thing now is to avoid any stress on your little guy.DO not suggest he does anything to him, he is the Vet and it sounds like he has been treating him for a long time.We all get old alas and the body just gets tired and starts to wear out,just love him and know he has had a wonderful life with you. Your vet might suggest a few things to prolong his life
but make sure it is quality of life,it is a fine line between keeping him alive for yourself or for him......know the difference.
Please let us know how he is going.
It sounds possibly as if your dogs stomach has flipped over. My boxer was like this and his vet couldn't see it on the x-ray. Please have them tube your dogs stomach to release the air and his stomach will roll back over.