Aw, I am sorry your dog's health is going downhill. They do sell sling things you can put under his tummy so you can help support his back end. They probably make disposable doggy diapers to help with the bowel control problem. It might help you get prepared for the inevitable if you can have just a little more time with him while he still seems comfortable enough. You are fortunate that he lived such a long life. You must have cared for him very well over the years and had luck on your side, and you are to be commended. But the loss will be tough for you because he's been with you so long.
It mght be best to leave him in peace, as in a funny way he sounds as if he's not too bad.
But if he's a tough old thing, and doesn't mind too much being messed around at the vet's, you could investigate that tumor, it might be benign (harmless) Many are.
The loss of bowel control....have you spoken to the vet about this, and do you know why this is happening? Does he have a condition which has caused this, such as disk problems causing paralysis? Or is it a diarrhea problem?
It could be that he has arthritis in his back or hips, and by the time he realizes he has to go poop, he can't get moving fast enough, and it's too late. Or he may have trouble squatting to poop....?
Arhritic conditions would probably respond somewhat to anti-inflammatory or pain reducing medications.
Sounds like he is doing the best he can. If the dog is eating and doing OK. I know a Vet could tell you more, but understand the economy makes it tough to see a Vet sometimes.